THE  JOHNS  HOPKINS  HOSPITAL  REPORTS 
MONOGRAPHS.    NEW  SERIES  No.  V 


THE   ORIGIN  AND  DEVELOPMENT  OF 
THE  LYMPHATIC  SYSTEM 

BY 
FLORENCE  R.  JSABIN 

(From  the  Anatomical  Laboratory  of  The  Johns  Hopkins  University,  Baltimore) 


BALTIMORE 

THE  JOHNS  HOPKINS  PRESS 
1913 

(Copyright,  1913,  by  Th«  Johns  Hopkinc  Prew! 


CONTENTS. 

PAGE 

I.  Introduction    1 

1.  Morphology  of  the  vascular  system,  the  angioblast 1 

2.  Morphology  of  the  lymphatic  system 4 

II.  Historical    4 

1.  Vasa  serosa 4 

2.  Lymphatics  with  open  mouths 4 

3.  Discovery  of  endothelium 6 

III.  Embryology    7 

1 .  Introduction 7 

2.  Lymphatics  as  dilated  tissue  spaces 8 

3.  Analysis  of  Budge's  work 8 

4.  Statement  of  the  three  arguments  for  the  origin  of  lymphat- 

ics from  the  veins 10 

A.  Growth  of  lymphatic  capillaries  by  sprouting 10 

B.  Lymphatic  and  non-lymphatic  zones 18 

C.  Direct  budding   of   the    lymphatic   vessels   from   the 

veins    21 

IV.  Primary   lymphatic   system   in   different   forms    24 

1.  Primary  lymphatic  sacs  in  the  pig 24 

A.  General  summary  of  the  sacs 24 

B.  Origin  of  the  jugular  sacs 25 

C.  Origin  of  the  renal  sacs 35 

a.  The  retroperitoneal  sac 35 

b.  The  iliac  sacs 36 

c.  The  cisterna  chyli 36 

D.  The  thoracic  duct 40 

2.  Primary  lymphatic  sacs  in  human  embryos 45 

3.  Primary  lymphatic  sacs  in  rabbit  embryos 47 

4.  Primary  lymphatic  sacs  in  cat  embryos 48 

5.  Primary  lymphatic  system  in  birds,  posterior  lymph  heart 

and  jugular  lymph  sac 50 

6.  Primary  lymphatic  system  in  amphibia,  anterior  and  poster- 

ior lymph  hearts 55 

V.  Peripheral  lymphatic  system 59 

1.  In  the  pig 59 

A.  From  the  jugular  sacs 59 

B.  From  the  renal  sacs 63 

2.  In  other  forms 65 

VI.  Origin  and  development  of  lymphatic  glands  and  their  relation 

to  primary  lymph  sacs  65 


11  CONTEXTS. 

PAGE 
VII.  Comparative  morphology  of  primary  lymph  sacs,  lymph  hearts, 

lymph  glands  and  amphibian  lymph  sacs 70 

VIII.  Various  other  theories  in  regard  to  the  origin  and  development 

of  the  lymphatic  system 72 

1.  Mayer-Lewis    Anlagen    72 

2.  Extra-intimal  and  perineural  spaces  and  f enestration 75 

3.  Growth  of  lymphatics  by  the  addition  of  tissue  spaces 77 

IX.  Conclusions    82 

X.  Literature  . .  83 


THE  ORIGIN  AND  DEVELOPMENT  OF  THE  LYMPHATIC 
SYSTEM.1 

By  FLORENCE   R.  SABIN. 

(From  the  Anatomical  Laboratory  of  the  Johns  Hopkins  University, 
Baltimore.} 


I.  INTRODUCTION. 

1.    MOKPHOLOGY     OF     THE    VASCULAR     SYSTEM,     THE    AxGIOBLAST. 

Until  the  past  few  years  our  knowledge  of  the  morphology  of  the 
lymphatic  system  was  in  a  very  unsatisfactory  state.  Our  con- 
ceptions of  such  main  questions  as  the  origin  of  the  first  lymphatics, 
the  time  of  their  appearance,  the  relations  of  the  lymph  hearts 
and  sacs  of  the  amphibia  to  the  lymph  glands  and  ducts  of  higher 
forms,  the  relation  of  the  lymphatics  to  serous  cavities  and  to  the 
various  forms  of  tissue  spaces,  and  the  development  of  the  lym- 
phatic vessels  within  an  organ  have  been  so  vaguely,  understood 
that  the  opinions  of  no  two  investigators  approached  agreement. 

A  clear  conception  of  any  system  must  be  based  on  an  under- 
standing of  its  fundamental  morphology,  as,  for  example,  the 
morphology  of  the  arterial  system  was  placed  upon  a  satisfactory 
basis  by  the  discovery  and  comparison  of  the  aortic  arches  in  dif- 
ferent animals.  An  example  more  closely  in  touch  with  our  knowl- 
edge of  the  lymphatic  system  is  the  study  of  the  fundamental 
morphology  of  the  blood  vascular  system.  Our  knowledge  of  this 
is  in  the  process  of  being  built  up,  but  a  foundation  has  been  laid 
for  the  clear  understanding  of  the  vascular  system  in  a  series  of 
discoveries.  The  first  and  most  fundamental  of  these  is  that  the 
blood  vessels  arise  as  blood  islands  in  the  extraembryonal  mem- 
branes. The  earlier  embryologists  on  the  other  hand  believed  that 
the  first  vessels  were  spaces  without  walls,  which  the  pressure  of 
the  circulating  blood  hollowed  out.  The  best  description  of  the 

1  Aided  by  a  grant  from  the  Baltimore  Association  for  the  Promotion 
of  University  Education  of  Women  and  by  the  Carnegie  Institution  of 
Washington. 


2  Florence  R.  Sabin. 

origin  of  blood  islands  and  the  best  analysis  of  the  meaning  of  this 
discovery  is  to  be  found  in  two  works  of  His,  "  Untersuchungen 
ueber  die  erste  Anlage  des  Wirbeltierleibes "  in  1868  (pp.  95-103) 
and  in  "  Lecithoblast  und  Angioblast"  in  1900  (pp.  268-295).  The 
discovery  of  blood  islands,  however,  dates  back  to  the  work  of 
Wolff  (154)  and  Pander  (101),  who  introduced  the  name,  and 
perhaps  no  one  subject  in  embryology  has  a  more  extensive  litera- 
ture. Certainly  a  most  interesting  account  of  the  development  of 
the  vascular  problem  can  be  followed  through  the  pages  of  von 
Baer  (7),  Prevost  and  Lebert  (108),  Eemak  (124-125),  Eeichert 
(121-123),  Koelliker  (67-71)  and  Thoma  (148).  The  second 
great  advance  was  the  discovery  that  blood  vessels  throughout  are 
lined  by  endothelium  (Hoyer  48),  which  followed*  soon  after  the 
corresponding  discovery  in  lymphatics  by  von  Becklinghausen.  The 
third  discovery  involves  the  proof  that  blood  vessels  grow  by  the 
sprouting  of  their  endothelium,  Prevost  and  Lebert  (108),  His 
(47),  Eouget  (126)  and  Arnold  (5) ;  the  fourth  that  the  main 
vessels  of  the  body  wall,  including  the  posterior  part  of  the  aorta 
(Evans  33-35),  even  the  anterior  part  of  the  aorta  and  lateral  heart 
anlagen,  arise  as  a  capillary  plexus  or  as  solid  angioblast  cords 
(Bremer  15),  which  invade  the  body  from  the  extraembryonal  mem- 
branes. Finally  a  complete  conception  of  the  development  of  the 
vascular  system  is  based  on  the  theory  that  the  blood  vessels  of  the 
extraembryonal  membranes  invade  the  body  wall  (His  47),  and 
that  within  the  body  wall  these  capillaries  of  endothelium  gradu- 
ally invade  or  spread  over  the  body  so  that  there  are  definite  vascu- 
lar and  non-vascular  layers  and  zones.  His  (47)  discovered  the  fact 
that  blood  vessels  grow  into  the  central  nervous  system ;  but  the  theory 
of  vascular  and  non-vascular  zones  which  is  essential  to  an  understand- 
ing of  the  development  of  the  vascular  system  we  owe  to  Mall  (80-87). 
It  has  been  worked  out  by  his  pupils,  notably  Evans  (33-35).  Its 
meaning  can  be  grasped  by  studying  figs.  4,  5  and  6  in  connection 
with  fig.  437  (Evans  35),  which  all  show  that  the  skin  dorsal  to  the 
central  nervous  system  is  a  non-vascular  zone,  which  for  a  long 
time  is  not  reached  by  the  blood  vessels  in  the  centrifugal  growth. 
This  series  of  discoveries,  notwithstanding  the  gaps  and  uncer- 
tainties in  our  knowledge  of  the  early  stages  which  were  well 
brought  out  by  Minot  (98,  pp.  483-485)  and  by  Evans  (35,  pp.  551- 
567)  in  1911,  uncertainties  in  part  since  removed  by  the  work  of 


Tlie  Origin  and  Development  of  the  Lymphatic  System.         3 

Bremer  (15),  offers  a  consistent  as  well  as  a  constructive  theory 
of  the  vascular  system  which  goes  far  to  modify  the  force  of  the 
following  words  of.Koelliker  quoted  by  Eiickert  and  Mollier  (128, 
p.  1019):  "Ueber  die  erste  Bildung  der  (Masse  und  des  Blutes 
herrschen  wohl  ebenso  viele  Ansichten,  als  Forscher  sich  ueber 
diesen  Gegenstand  ausgesprochen  haben,  was  auf  jeden  Fall  be- 
weist,  dass  derselbe  zu  den  schwierigsten  gehort."  The  theory 
thus  outlined  is  that  the  fundamental  morphology  of  the  vascular 
system  is  based  on  the  specificity  of  the  endothelium,  or  in  the 
words  of  His  (47,  p.  325),  on  the  fact  "Zu  den  am  friihesten  sich 
sondernden  Gewebsanlagen  gehort  der  Gefasskeim  oder  Angioblast. 
Seine  Sonderung  erfolgt  sehr  scharf,  und  sein  Wachsthum  geht 
nach  durchaus  eigenthumlichen  Gesetzen  vor  sich."  It  includes 
the  discovery  of  Mall  (83-87)  that  endothelium  may  produce  reticu- 
lum,  a  process  which  he  has  shown  takes  place  both  in  the  liver 
and  in  the  heart;  but  maintains  that  there  is  an  early  differentia- 
tion of  two  tissues,  namely,  endothelium  and  mesenchyme,  so  that 
the  angioblasts  once  formed  give  rise  to  all  the  vascular  endothelium 
of  the  body. 

The  opposing  theories  in  the  varying  forms  of  the  origin  of  blood 
vessels  from  tissue  spaces,  their  growth  by  the  addition  of  tissue 
spaces  or  by  the  addition  of  connective  tissue  cells,  and  the  differ- 
entiation of  endothelium  from  naesencihyme  over  extensive  areas 
within  the  body  wall  find  their  most  recent  evidence  in  the  works 
of  Eiickert  (127),  Eiickert  and  Mollier  (128),  Hahn  (40),  Bon- 
net (14)  and  Maximow,  1909  (90,  p.  511).  These  views,  however, 
must  be  traced  through  the  earlier  works  of  Gotte  (37)  and  Beich- 
ert  (121-123).  The  evidence  for  the  continued  origin  of  blood 
vessels  from  the  mesenchyme  is  for  the  most  part  from  the  inter- 
pretation of  appearances  in  sections.  Hahn's  work,  however,  in 
connection  with  the  difficult  point  of  the  origin  of  the  heart  and 
aorta  is  experimental.  He  removed  the  vascular  membranes  on 
one  side  of  early  chick  embryos  and  obtained  a  heart  anlage  and 
aorta  on  both  sides.  He  thought  that  he  could  entirely  exclude  a 
growth  from  one  side  to  the  other  as  well  as  remnants  of  the  mem- 
branes on  the  same  side  and  concluded  that  both  heart  and  aorta 
arise  in  situ  from  the  mesenchyme  and  not  from  the  endoderm  of 
the  yolk  sac  not  from  an  ingrowth  from  the  extraembryonal  mem- 
branes. 


4  Florence  R.  Sabin. 

2.  MORPHOLOGY  OF  THE  LYMPHATIC  SYSTEM. 

It  will  be  proved  in  the  following  pages  that  the  lymphatic  problem 
is  closely  connected  with,  or  rather  is  a  fundamental  part  of,  the 
vascular  problem,  so  that  the  study  of  the  one  throws  light  on  the 
other,  or.  to  put  it  more  strongly,  that  the  same  kind  of  evidence  is 
needed  to  solve  both  problems.  The  fundamental  morphology  of  the 
lymphatic  system  has,  however,  been  put  on  a  more  satisfactory  basis 
than  that  of  the  vascular  system,  for  it  has  been  seen  in  the  living 
embryo  that  the  first  lymphatics  bud  off  from  the  veins.  Moreover, 
lymphatics  begin  at  a  comparatively  late  stage,  long  after  the  forma- 
tion of  the  blood  islands  has  ceased. 

* 

II.  HISTORICAL. 

1.  VASA  SEROSA. 

Until  the  primary  origin  of  any  system  is  known  our  conceptions  of 
it  are  necessarily  hazy  and  vague  and  this  is  nowhere  better  illus- 
trated than  in  connection  with  the  lymphatic  system.     The  views  of 
I  the  eighteenth  century  may  well  be  summed  up  in  the  hypothetical 
\  "vasa  serosa  "  of  Boerhave  (13),  Haller  (41)  and  others,  which  were 
J  tiny  channels  too  small  to  allow  corpuscles  to  pass,  supposed  to  connect 
T    arteries,  veins  and  lymphatics  at  their  tips.     The  conception  which 
7  underlies  vasa  serosa  may  be  traced  back  to  the  experiments  of  Xiu-k 
l^{100),  who  injected  air  into  the  arteries  and  found  it  returning  in 
/the  lymphatics,  as  may  be  represented  in  the  diagram  of  fig.  1.    Vasa 
(serosa  meant  the  idea  that  the  arteries  finally  branched  into  vessels  too 
tinj  to  carry  the  corpuscles,  but  the  term  likewise  represents  a  whole 
series  of  vague  conceptions,  such  as  Bichat's  (12)  absorbents  and  ex- 
halants,  which  sought  to  make  definite  some  idea  of  the  nature  of 
lymphatics. 

2.  LYMPHATICS  WITH  OPEX  MOUTHS. 

In  the  latter  half  of  the  eighteenth  century  the  conception  of  definite 
"vasa  serosa"  was  modified  through  the  work  of  William  Hunter  (50) 
and  Munro,  who  believed  that  lymphatics  began  with  open  mouths. 
The  views  of  these  English  observers,  as  shown  by  Cruikshank  (23), 
involved  the  idea  that  the  mouths  of  the  lymphatics  opened  directly 
onto  the  surface  of  the  body,  into  the  cavity  of  the  intestine  and  the 
air  sacs  of  the  lungs,  as  well  as  into  the  connective  tissues.  The  theory 
was  also  involved  with  discussions  of  the  lymphatics  as  the  exclusive 


The  Origin  and  Development  of  the  Lymphatic  System. 


organs  of  absorption  for  the  body,  and  the  physiological  theories  of 
filtration. 


Flo.  1. — Diagram  to  show  the  "  vast  serosa  "  of  Boerhave,  Haller  and 
others.  The  veins  are  striped,  the  lymphatics  are  dotted,  and  the  "  vasa 
serosa  "  which  connect  them  are  cross-hatched.  The  connective  tissue  is 
indicated  by  lines. 

The  doctrine,  or  as  His  (44)  terms  it,  "  the  dogma  of  lymphatics 
with  open  mouths,"  was  first  combated  by  a  definite  counter  theory 
by  Schwann  (139),  following  his  discovery  of  the  cell  in  the  animal 
body.  The  theory  of  Schwann  and  notably  of  Yirchow  (150)  was  that 
in  place  of  vasa  serosa  and  hypothetical  connections  which  did  not 
exist  hollow  connective  tissue  cells  spanned  the  gaps  between  blood 
vessels  and  lymphatics,  fig,  2. 


FIG.  2. — Diagram  to  show  the  theory  of  Schwann  and  of  Virchow  that 
the  blood  vessels  are  connected  with  lymphatics  by  hollow  connective  tissue 
cells.  The  veins  are  striped,  the  lymphatics  dotted  and  the  hypothetical 
lymph  vessels  are  cross-hatched.  The  nuclei  denote  the  fact  that  cells  have 
been  discovered. 


6  Florence  R.  Sabin. 

3.  DISCOVERY  OF  EXDOTHELIOI. 

In  contrast  to  these  hypothetical  relationships  of  lymphatics,  the 
work  of  von  Becklinghausen  (119)  gave  the  first  definite  theory  which 
I  was  actuallv  based  on  the  interpretation  of  direct  observations  of  the 
I  lymphatic  capillary.  Through  the  use  of  silver  nitrate  solutions  re- 
duced in  direct  sunlight,  he  discovered  that  the  lymphatics  are  lined  by 
a  definite  cell  layer  of  endothelium.  This  is  clearly  one  of  the  most 
important  discoveries  in  histology.  Von  Eecklinghausen  interpreted 
the  silver  pictures  to  mean  that  the  finest  lymphatic  capillaries  were 
lined  by  an  epithelium,  but  that  these  capillaries  communicated  by 
wide  openings  with  Saftca-nale  or  lymph  radicles,  which  formed 
the  real  rootlets  of  the  lymphatics.  Subsequently  h^  described  more 
definite  openings  or  stomata  in  the  serosa  of  the  diaphragm  (120). 


FIG.  3. — Diagram  to  show  von  Recklinghausen's  theory  of  the  relation  of 
lymphatics  to  tissue  spaces.  All  of  the  tissue  spaces  are  cross-hatched  to 
indicate  hypothetical  lymphatics. 

The  von  Recklinghausen  figures  seemed  to  offer  a  perfectly  satisfac- 
tory demonstration  of  the  relations  of  open  lymphatics,  especially  in 
view  of  the  fact  that  the  great  majority  of  anatomists  already  believed 
in  open  lymphatics.  But  it  is  of  great  interest  to  follow  the  writings 
of  His  at  this  period.  His  (44),  who  was  just  publishing  in  favor  of 
open  lymphatics,  and  saw  von  Eecklinghausen's  paper  as  his  own  was 
going  to  press,  at  first  recognized  in  von  Recklinghausen's  descriptions 
a  confirmation  of  his  own  views.  On  careful  studying  of  the  silver 
pictures,  however,  His  (45)  saw  that  the  discovery  of  endothelium  was 
the  very  step  which,  instead  of  proving  that  lymphatics  are  open  and 


The  Origin  and  Development  of  the  Lymphatic  System.         7 

without  definite  limits,  tended  rather  to  throw  the  balance  of  proof  in 
the  opposite  direction ;  that  the  discovery  of  endothelium  gave  the  con- 
ception of  lymphatic  capillaries  not  as  indefinite  spaces  without  walls, 
but  as  limited  by  a  definite  membrane.  To  quote  his  words  (45,  pp. 
457-458),  "  Wenn  ich  nach  den  eben  gemachten  Auseinandersetzungen 
der  einen  Aufstellung  v.  Recklinghausen's,  namlich  derjenigen  vom 
Vorkommen  eines  Epithels  in  den  feineren  Lymphgefassen  des 
Diaphragma  vollkommen  beistimme,  so  bin  ich  durchaus  anderer 
Ansicht  als  jener  Autor  hinsichtlich  der  Saftcanalchen  und  ihrer  Bezie- 
hung  zu  den  Lymphgefassen ;  f iirs  erste  namlich  leugne  ich  das  Vor- 
kommen von  Saftcanalchen  in  dem  Sinn  von  v.  Eecklinghausen,  und 
zweitens  halte  ich  die  Verbindung  der  von  ihm  als  Saftcanalchen 
gedeuteten  Theile  mit  den  Lymphgefassen  nur  fur  eine  scheinbare," 
and  later  in  his  interpretation  of  the  silver  picture  he  says :  "  Je 
du'nner  und  blasser  die  supervasculare  Schicht  ist,  um  so  leichter  wird 
es  den  Anschein  haben,  als  ob  die  hellen  Figuren  der  intervascularen 
Bindegewebskb'rper  unmittelbar  in  die  Lymphgefasse  selbst  einmunde- 
ten,  um  so  eher  wird  man  uebersehen,  dass  in  Wirklichkeit  der  Zusam- 
menhang  jener  Figuren  nicht  mit  den  Gefassen,  sondern  mit  den 
Gefassen  ueberlagerten  verzweigten  Korpern  stattfindet."  Further  he 
adds  that  if  the  openings  pictured  by  von  Eecklinghausen  exist,  there 
would  be  no  reason  why  any  injection  of  a  finely  divided  mass  should 
not  fill  practically  all  of  the  Saftcanalchen  or  tissue  spaces  as  ex- 
pressed in  fig.  3,  and  this  is  obviously  not  the  case.  Thus  after  a  care- 
ful study  and  interpretation  of  the  silver  reaction  on  the  lymphatic 
vessels,  on  the  albuminous  fluid  of  the  interspaces,  and  on  the  cell 
bodies  of  the  connective  tissue  corpuscles,  His  was  led  to  believe  that 
the  significance  of  the  von  Eecklinghausen  discovery  was  that  lym- 
phatics are  lined  throughout  by  an  endothelium  rather  than  that  the 
silver  method  demonstrates  open  lymphatics. 

III.  EMBRYOLOGY. 
1.  INTRODUCTION. 

It  is,  I  think,  clear  that  the  problem  in  this  state  could  only  be 
attacked  successfully  by  embryology  with  the  purpose  of  establishing 
the  fundamental  morphology  of  the  system.  The  importance  of  this 
is  best  expressed  by  His  (44,  p.  223),  "Von  den  mancherlei  offenen 
Fragen,  die  seit  mehr  denn  200  Jahren  in  den  anatomischen  Schriften 


8  Florence  R.  Sabin. 

ueber  das  Lymphsystern  discutirt  zu  werden  pflegen,  bietet  wohl  keine 
ein  so  unmittelbares  pli3rsiologisches  Interesse  dar,  als  gerade  die  Frage 
nach  dem  Ursprung  der  Lymphgefasse  in  den  Korperorganen.  Mag 
man  sich  ueber  die  Bildung  der  Lymphe  und  ueber  die  bei  ihrer 
Fortbewegung  wirksamen  Krafte  eine  Theorie  machen,  welche  man 
will,  so  fordert  diese  als  ganz  unerlassliche  Grundlage  eine  praezisere 
Vorstellung  von  dem  anatomischen  Verhalten  der  ersten  ^Yu^zeln  des 
Systems,  sei  nun  eine  solche  Vorstellung  wirklich  aus  der  Beobachtung 
entsprossen,  sei  sie  nur  hypothetisch  angenommen."  It  is  without 
question  that  a  comprehension  of  the  relations  of  the  lymphatic  capil- 
laries depends  on  an  understanding  of  the  fundamental  morphology 
of  the  system.  \ 

2.  LYMPHATICS  AS  DILATED  TISSUE  SPACES. 

The  relation  of  the  lymphatics  to  tissue  spaces  formulated  by  von 
Eecklinghausen  was  entirely  in  agreement  with  the  early  crude  investi- 

/gations  of  fcetal  conditions  recorded  by  Breschet  (16).  It  was  the 
practically  universally  accepted  idea  up  to  1900  that  lymphatics  arose 
by  the  dilatation  of  tissue  spaces  caused  by  the  fluid  that  exuded  from 
the  blood  vessels;  that  this  dilatation  of  the  spaces  began  in  the 
periphery,  and  that  the  vessels  gradually  approached  the  veins  and 
joined  them.  Thus  the  growth  of  the  lymphatic  vessels  was  thought 
to  be  from  the  periphery  to  the  center.  This  view  was  well  stated  in 
1894  by  Gulland  (39),  who  found  the  lymphatics  first  in  the  subcu- 
taneous tissue  and  then  along  the  extremities  in  foetuses,  human, 
bovine  and  rabbit,  between  3  and  4  cm.  long,  and  thought  that  these 
vessels  subsequently  joined  the  veins.  He  thought  that  the  force  of 
the  fluid  in  the  tissues  formed  the  lymphatics,  so  that  they  arose  by  an 
entirely  different  method  from  his  conception  of  the  origin  of  blood 
vessels  (p.  467). 

3.  BUDGE'S  WORK. 

An  account  of  the  newer  work  on  the  origin  of  the  lymphatic 
system  must  begin  with  Budge  (17-22).  Budge  was  led  to  a  study  of 
the  lymphatic  system  in  birds  through  noticing  the  great  variations 
of  the  posterior  lymph  hearts  in  the  adult  (21).  The  posterior  lymph 
hearts  had  been  seen  in  birds  by  Panizza  (103),  A.  F.  J.  ilayer 
(quoted  by  Stannius)  and  Stannius  (142).  Budge  had  already  begun 


The  Origin  and  Development  of  the  Lymphatic  System.         9 

to  study  lymphatics  in  the  adult  liver  (17)  and  in  bone  (18)  by  means 
of  injection,  and  in_188.0  (19)  he  described  studies  of  a  system  of 
canals  which  he  could  inject  in  mesoderm  of  three-day  chick  embryos. 
These  injections,  which  can  be  easily  repeated,  giving  the  exact  pat- 
terns of  Budge,  were  really  made  into  the  extraembryonic  coelom,  as 
has  been  shown  by  Mall  (81)  and  myself  (129),  and  have  nothing  to 
do  with  the  lymphatics.  They  are  not,  as  Budge  himself  noted,  lined  \ 
by  endothelium ;  indeed  Budge  himself  was  most  cautious  in  his  inter-  \ 
pretation,  for  he  says  (19,  p.  325),  "  Mit  Absicht  habe  ich  den  Aus- 
druck  Lymphgefassystem  fur  das  eben  beschriebene  Canalsystem 
vermieden,  weil  mir  hierfiir  noch  nicht  Anhaltspunkte  geiiug  gewon- 
nen  zu  sein  scheinen.  Und  doch  liegt  der  Gedanke  hieran  nahe."  In 
1881  Budge  (20)  announced  the  important  discovery  that  there  was 
a  plexus  of  lymphatic  vessels  accompanying  the  allantoic  arteries  of 
chicks  easily  injected  from  the  sixteenth  to  the  eighteenth  day  of  incu- 
bation, and  that  these  injections  ran  to  the  thoracic  duct.  The' next 
year  (21)  in  a  fuller  paper  he  described  that  these  allantoic  lymph- 
atics not  only  entered  the  thoracic  duct  but  drained  into  the  posterior 
lymph  heart,  which  played  a  great  role  in  relation  to  the  allantois. 
He  injected  the  lymph  heart  through  the  allantoic  vessels  by  the  tenth 
day,  and  noted  that  the  posterior  lymph  heart  was  preceded,  as  seen 
in  the  living  chick  (8  to  12  days),  by  veins,  and  that  subsequently  the 
lymph  heart  appeared,  which  could  be  distinguished  by  the  fact  that  it 
pulsated  with  a  different  rhythm  from  the  blood  vessels.  After 
Budge's  death  his  work  was  brought  together  by  His  (22)  and  formu- 
lated somewhat  in  this  manner:  There  are  two  lymphatic  systems,  a 
transitory  and  a  permanent  system,  the  first  consisting  of  spaces  in  the 
extraembryonal  membranes  analogous  to  the  coelom  and  found  only  in 
early  stages.  The  second,  consisting  of  the  true  endothelial-lined 
vessels  which  accompany  blood  vessels — which  Budge  so  successfully 
injected.  The  thoracic  duct  he  thought  arose  from  the  first  system  and 
was  the  only  permanent  part  of  the  first  system.  Thus  Budge's  work, 
which  was  a  reaching  out  into  a  dark  field  and  had  genuine  important 
discoveries,  was  in  line  with  the  prevailing  theory  that  lymphatics  are 
related  to  tissue  spaces,  and  introduced  a  misleading  conception  that 
lymphatics  form  after  the  same  manner  as  the  coelom  and  indeed  in 
part  as  an  extension  of  the  ccelom  as  far  as  the  primativ  system  and 
thoracic  duct  are  concerned. 


10  Florence  R.  Sabin. 

4.  STATEMENT  OF  THE  ARGUMENTS  FOR  THE  VENOUS  ORIGIN  OF  THE 
LYMPHATIC  SYSTEM. 

I  shall  now  take  up  the  proof  of  the  theory  that  the  lymphatic 
system  is  derived  from  the  venous  system,  and  will  subsequently 
analyze  the  opposing  theory,  which,  in  its  most  recent  from,  is  that 
lymphatics  grow  on  the  addition  of  tissue  spaces  (Kampmeier  66  and 
66a). 

The  essential  elements  in  the  establishing  of  the  venous  origin  of 
the  lymphatics  have  been  (1)  the  proof  that  lymphatic  capillaries 
grow  by  the  sprouting  of  their  endothelial  wall  and  not  by  the  addition 
or  hollowing  out  of  connective  tissue  spaces.  This  argument  is  known 
as  the  method  of  growth  by  sprouting.  (2)  The  <proof  that  lym- 
phatics gradually  invade  the  body  from  center  to  periphery,  establishing 
the  point  of  lymphatic  and  non-lymphatic  zones.  This  means  that 
the  main  ducts  grow  as  capillaries  by  the  same  method  as  the  per- 
ipheral capillaries.  This  argument  is  known  as  the  one  of  lymphatic 
and  non-lymphatic  zones.  (3)  That  the  original  lymphatic  ducts 
bud  off  directly  from  the  veins.  This  is  known  as  the  origin  of  the 
lymph  sacs  by  budding. 

"~  Thus  the  study  involves  three  elements :  The  nature  of  the  lym- 
phatic capillary,  the  nature  of  the  lymphatic  ducts,  and  the  nature  of 
the  first  lymphatics  or  the  lymph  sacs.  Each  one  of  these  three  lines  of 
proof  involves  the  study  of  endothelium ;  the  first  lymphatics  are  buds 
from  the  endothelial  lining  of  the  veins ;  these  endothelial  buds  invade 
the  body  down  to  the  ultimate  capillaries  by  the  cell  division  of  their 
endothelial  wall. 

A.    GROWTH  OF  LYMPHATIC  CAPILLARIES  BY  SPROUTING. 

A  study  of  the  method  of  growth  of  lymphatic  capillaries  is  neces- 
sarily associated  with  the  study  of  the  method  of  growth  of  blood 
capillaries,  for  as  far  back  as  the  time  of  Schwann  (139),  who  first 
saw  the  capillaries  in  the  living  tadpole's  tail  and  who  vaguely  dis- 
tinguished those  which  carried  blood  from  those  which  had  no  cor- 
puscles, there  is  a  clear  recognition  that  the  two  forms  of  capillaries 
grow  by  the  same  method.  This  method  Schwann  thought  to  be  by 
the  addition  of  hollow  connective  tissue  cells. 

The  term  growth  by  sprouting  was  introduced  quite  recently  by  His, 
but  the  first  description  of  the  process  dates  back  to  the  year  1844.  to 


The  Origin  and  Development  of  the  Lymphatic  System.       11 

the  work  of  Platner  (106)  and  of  Prevost  and  Lebert  (108).  Plainer 
described  the  growth  of  capillaries  as  seen  in  the  living  tadpole's  tail. 
He  analyzes  Schwann's  idea  of  the  addition  of  cells  as  necessarily 
merely  an  inference  from  the  appearance  of  the  vessels  in  one  stage,  for 
when  actually  seen  growing  new  cells  are  never  added  on.  He  says 
that  rather  each  new  vessel  is  a  process  of  a  preceding  one  and 
describes  these  processes  as  long,  thin  Auslaufer,  which  form  loops 
and  soon  show  a  double  contour.  He  says  that  the  tiny  processes 
never  contain  nuclei  and  are  not  cells,  so  that  the  interpretation  of 
Schwann  of  the  addition  of  cells  could  not  possibly  hold.  Platner 
says  that  the  same  process  of  growth  in  the  living  tadpole  had  been 
observed  by  Prevost  and  Lebert.  Indeed,  in  the  same  year  (108)  they 
published  a  series  of  four  papers,  in  which  they  describe  the  growth 
of  capillaries,  not  only  in  Batrachians,  but  more  in  detail  in  connec- 
tion with  the  blood  islands  in  the  chick.  They  definitely  use  the  term 
centrifugal  growth  and  describe  the  process  in  the  following  graphic 
terms  (p.  239):  "  Les  vaisseaux  poussent  des  saillies  laterales  par 
decollement  partiel  des  lamelles  du  feuillet  vasculaire,  saillies  plus 
ou  moins  arrondies  ou  pointues,  allongees,  formant  des  especes 
d'eperons  qui  souvent  fmissent  par  se  rencontrer,  provenant  de  deux 
cotes  diffe  rents,  et  etablissent  ainsi  des  vaisseaux  de  communication." 
They  thought  that  the  processes  or  "  spurs  "  were  for  the  most  part 
hollow  from  the  start  and  state  that  the  larger  vessels  grew  by  the 
same  method  as  the  smaller. 

In  1846  Koelliker  published  two  papers  (67  and  68),  in  one  of 
which  he  announced  the  discovery  of  lymphatic  capillaries  in  the 
tadpole's  tail  (67).  He  described  the  characteristic  differences  as  seen 
in  the  living  form  between  the  two  types  of  capillaries,  the  irregulari- 
ties and  more  numerous  processes  of  the  lymphatics  as  well  as  their 
connection  with  the  superior  and  inferior  caudal  lymph  trunks.  In 
the  other  paper  he  gives  a  most  valuable  analysis  of  the  views  of  the 
early  embryologists,  Wolff,  von  Baer,  Eeichert  and  others  on  the  prob- 
lem of  the  growth  of  vessels.  He  takes  up  the  various  views  of  growth 
(p.  118)  which  were  then  being  discussed,  namely,  growth  (1)  by  the 
addition  of  unbranched  cells,  or  (2)  by  branched  or  stellate  cells, 
or  (3)  by  spaces  hollowed  out  by  the  force  of  the  heart  beat,  or  (4) 
from  spaces  produced  by  fluids  in  the  tissues  as  well  as  (5)  by  the 
then  new  work  of  Platner,  Prevost  and  Lebert,  showing  centrifugal 


12  Florence  R.  Sabin. 

growth,  and  concludes  that  the  vascular  and  lymphatic  capillaries 
grow  by  the  addition  of  branched  or  star-shaped  cells. 

Four  years  later,  1850,  Kemak  (125)  rediscovered  the  lymphatics 
in  the  tadpole's  tail  by  watching  the  living  embryo,  and,  without 
knowing  of  the  preceding  work,  he  hurriedly  published  an  even  more 
graphic  description  of  the  form  and  growth  of  lymphatics  than  Koel- 
liker's.  Subsequently,  after  seeing  Koelliker's  work,  he  added  a 
note  (p.  182-183)  to  emphasize  the  fact  that  if  the  growth  be  watched 
in  the  living  form  star-shaped  cells  are  never  added — neither  to  the 
blood  capillaries,  nor  to  the  lymphatic  capillaries,  nor,  he  adds,  to  the 
growing  nerves.  Many  years  later,  1886,  Koelliker  (71,  p.  3-i),  in 
an  article  mainly  on  nerves,  corrected  his  view  of  {he  growth  of  the 
lymphatics  by  the  addition  of  connective  cells  in  favor  of  the  theory 
of  growth  by  sprouting. 

The  years  from  1863  to  1867  represent  a  great  advance  in  our  con- 
ceptions of  the  h-mphatic  system.  The  historical  article  of  His  (44) 
on  the  lymphatic  system  must  form  the  basis  of  any  work  which  deals 
with  the  development  of  the  problem.  In  1865  Strieker  (143)  pub- 
lished a  paper  in  which  he  expresses  a  doubt  in  regard  to  Koelliker's 
discovery  of  lymphatics  in  the  tadpole's  tail,  on  account  of  the  fact  that 
undoubted  blood  capillaries  had  also  the  blind  sprouts.  He,  however, 
submitted  the  blood  capillaries  to  an  experimental  study  of  great 
interest.  He  stimulated  them  with  the  vapor  of  ammonia  and  gives 
the  following  description  of  his  results: 

Ich  sah  ein  Gefassstiick,  welches  seinem  Baue  nach  unzweifelhaft  zu  den 
Kapillaren  gezahlt  werden  musste,  sich  im  Laufe  von  fiinfzehn  Minuten 
zweimal  bis  nahe  zum  Verschwinden  des  Lumens  verengern  und  bis  zum 
Breitendurchmesser  eines  Blutkorperchens  wieder  erweitern.  Ich  sah 
varicose  Gefasse  sich  so  veranderen,  dass  der  Bauch  an  Stelle  des  Halses  zu 
liegen  kam,  und  umgekehrt.  Mit  den  Formveranderungen  ging  gewohnlich 
noch  eine  andere  Veranderung  Hand  in  Hand.  So  wurde  beispielweise  eine 
eingeschniirte  Kapillargefassstrecke  so  blass,  wurden  deren  Conturen  so 
undeutlich,  dass  ich  sie  kaum  mehr  von  der  Umgebung  unterscheiden 
konnte,  wahrend  nach  bis  zwanzig  Minuten  das  Gefass  wieder  in  seiner 
urspriinglichen  Deutlichkeit  zu  sehen  war. 

Thus  he  discovered  the  contractility  of  endothelium.  He  found  that 
electricity  was  a  still  better  stimulant  for  observing  this  contractility. 
He  discusses  his  results  from  the  standpoint  of  determining  the  nature 
of  the  capillary  wall,  especially  in  connection  with  the  silver  prepara- 
tions of  Aeby,  Auerbach  and  Eberth  (quoted  from  Strieker  143), 


The  Origin  and  Development  of  the  Lymphatic  System.       13 

which  has  already  given  the  suggestion  of  a  lining  of  flattened  cells 
for  blood  capillaries.  Strieker  saw  clearly  that  the  wall  of  the  capil- 
lary was  of  living  protoplasm,  but  was  confused  in  regard  to  whether 
the  blood  stream  was  inter-  or  intra-cellular.  He  was  perplexed  also 
in  regard  to  the  existence  of  cells  or  not,  since  he  was  dealing  with 
the  growing  tip,  which  we  now  know  to  be  a  syncytium.  This  idea 
occurred  to  Strieker,  as  is  plain  in  these  words,  "  Dass  bei  dem  Auf  bau 
von  Kapillargefassen  ueberhaupt  Zellen  mit  einander  verschmelzen, 
und  dass  solche  Verschmelzungsspuren  spater  einmal  durch  Silber- 
niederschlage  kenntlich  werden  kb'nnen,  lasst  sich  ferner  auch  nicht 
leugnen."  Strieker  also  noted  the  sprouts  of  blood  vessels  as  indi- 
cating their  method  of  growth. 

I  have  already  taken  up  the  fact  that  von  Eecklinghausen's  work 
on  the  endothelium  of  lymphatic  vessels,  studied  by  means  of  the 
silver  nitrate  method,  settled  the  question  of  the  cellular  nature  of 
the  lining  of  lymphatics,  especially  in  connection  with  the  analysis  of 
the  work  by  His,  who  had  regarded  lymphatics  as  vessels  without 
walls  (44,  p.  229),  and  saw  by  a  thorough  study  of  preparations  made 
by  the  silver  method  that  lymphatics  were  lined  throughout  by  endo- 
thelium. 

In  1867  Langer  (75),  as  a  conclusion  to  a  series  of  studies  on  the 
lymphatic  system  in  amphibia,  published  a  study  of  the  lymphatics 
in  the  tadpole's  tail.  He  was  able  to  make  complete  injections  of  the 
blood  vessels  and  the  lymphatics  and  gives  a  .most  interesting  analysis 
of  specimens  as  observed  in  alcohol  from  the  standpoint  of  weighing 
the  evidence  on  the  method  of  growth.  He  describes  the  uninjected 
capillary  as  often  appearing  to  have  a  single  contour,  while  the  injected 
one  had  the  complete  wall.  He  could  not  find  in  the  alcoholic  speci- 
men the  tiny  processes  previously  described  in  the  living  form.  He 
noted,  however,  the  larger  sprouts,  the  loops  and  blind  processes,  and 
says  that  the  nuclei  are  definitely  within  the  contour  of  the  wall  of 
the  capillary,  so  that  he  makes  the  distinction  between  the  vessel  and 
the  surrounding  tissue.  He  concludes  that  the  various  forms  of  the 
processes  must  indicate  the  process  of  centrifugal  growth  for  the  lym- 
phatic capillary  as  well  as  for  the  blood  capillary. 

Again  in  1873  the  growth  of  the  lymphatics  and  blood  vessels  by 
the  method  of  sprouting  was  observed  in  the  tadpole's  tail  and  very 
clearly  described  by  Eouget.  By  the  use  of  curare  he  was  able  to  keep 
the  larva  still.  He  also  emphasizes  the  great  number  of  tiny  filaments 


1-1  Florence  R.  Sabin. 

on  the  lymphatics  and  the  fact  that  they  never  join  with  the  con- 
nective tissue  cells. 

Thus  the  observation  of  the  growth  of  capillaries  by  sprouting 
was  made  over  and  over  again  where  there  was  the  chance  of  seeing 
the  process  in  the  living  form  or  of  obtaining  injected  specimens, 
and  yet  during  all  these  years  the  observations  were  not  generally 
accepted,  since  as  the  fundamental  morphology  of  the  lymphatic  sys- 
tem was  unknown,  the  significance  of  the  observations  could  not  be 
tinderstood. 

It  is,  however,  not  true  that  every  observer  who  studied  the  living 
tadpole's  tail  described  the  process  of  the  sprouting  of  the  endothelial 
wall  of  blood  vessels  and  of  lymphatics  as  showing  the  method  of 
growth.  Indeed  at  least  three  observers  have  doubted  whether  there 
are  two  different  kinds  of  capillaries  in  the  tadpole's  tail.  I  have 
already  noted  that  Strieker  doubted  the  presence  of  lymphatic  capil- 
laries in  the  tadpole's  tail. 

Wysotzky  (155),  whose  work,  since  it  is  published  in  Eussian, 
I  quote  from  Mayer  (89),  thought  that  the  capillaries  without  blood 
were  merely  young  blood  capillaries. 

Sigmund  Mayer  (89),  in  an  interesting  and  valuable  paper,  "  Ueber 
die  Blutleerengefasse  im  Schwanze  der  Batrachierlarven,"  gives  first 
an  excellent  analysis  of  the  literature  on  the  subject  of  the  capillaries 
in  the  tadpole's  tail.  He  brings  out  the  fact  that  it  was  the  presence 
of  an  occasional  red  corpuscle  in  the  "  lymphatic "  which  was  the 
stumbling  block  in  accepting  the  presence  of  lymphatic  capillaries. 
He  then  records  an  observation  which  in  reality  cleared  up  the  entire 
difficulty  in  regard  to  the  "  empty  capillaries."  As  E.  E.  Clark  (26) 
has  shown,  Mayer,  however,  failed  to  see  the  bearing  of  his  own 
observations.  Mayer  used  curare  and  the  electric  current  to  anaes- 
thetize the  larva  and  then  covered  the  tail  with  a  cover  slip.  When  he 
sucked  the  water  out  from  under  the  cover  slip  he  noted  that  the 
blood  would  gradually  stop  flowing  or  even  go  in  the  reverse  direction 
in  the  blood  capillaries  and  that  the  vessel  would  collapse,  so  that  it 
looked  like  a  solid  cord.  Thus  he  also  observed  the  contraction  of 
endothelium.  Furthermore,  he  noted  that  the  "  other  empty  vessels  " 
(namely,  lymphatics),  contracted  more  frequently  than  the  blood 
capillaries.  When,  however,  he  studied  these  other  empty  vessels  he 
thought  that  the  occasional  cells  in  them  flowed  now  toward  the  center, 
now  toward  the  periphery,  and  he  ended  with  the  somewhat  vague 


The  Origin  and  Development  of  the  Lymphatic  System.       15 

and  erroneous  conclusion  that  the  empty  vessels,  though  they  might 
have  some  of  the  functions  of  the  lymphatics  of  the  adult,  could  not 
properly  be  grouped  with  them.  We  now  know  that  the  two  sorts  of 
empty  vessels  are  (1)  the  collapsed  blood  vessels  which  Mayer  himself 
observed  and  (2)  the  true  lymphatics  (E.  E.  Clark,  26). 

The  work  of  Mayer,  however,  has  another  point  of  great  interest 
to  us  beside  the  important  observation  of  the  contractility  of  endo- 
thelium,  namely,  what  we  call  Mayer-Lewis  anlagen.  Mayer  noted 
the  long  strands  of  cells  (endothelium)  which  often  span  the  gap 
between  two  capillaries;  indeed,  he  figures  them,  some  that  look 
entirely  solid  and  some  that  have,  here  and  there,  a  patent  lumen.  In 
reality  both  blood  capillaries  and  lypmhatic  capillaries  have  these 
strands  and  sprouts,  and  it  is  one  of  the  most  remarkable  qualities 
of  the  two  forms  of  endothelium  that  no  sprout  from  a  lymphatic 
capillary  ever  joins  a  blood  capillary  and  vice  versa.  However,  Mayer's 
mistaken  idea  that  these  strands  of  cells  connected  with  two  different 
kinds  of  capillaries,  namely,  the  vessels  which  are  always  empty,  in 
other  words,  the  lymphatics  and  the  blood  capillaries,  aided  in  his 
confusion  of  the  lymphatics  with  the  blood  vessels.  Besides  these 
strands  of  endothelium  he  saw  small  hollow  vessels,  having  the  same 
lining  as  the  capillaries,  which  he  thought  to  be  entirely  isolated. 
These  he  interpreted  as  follows  (p.  236)  : 

Unseren  Erfahrungen  nach  sind  nun  die  beschriebenen  Bilder  nur  gewisse 
Stadien  der  Ausschaltung  von  Bestandtheilen  des  Blutgefassystems  aus  der 

normalen  Circulation Bei  genauerem  Studium  der  letzteren  For- 

mationen  ergibt  sich,  dass  die  Wandungen  derartiger  mit  freien  Spitzen  in 
das  Gewebe  hineinragenden,  und  ausserhalb  jeder  Kommunication  mit 
Gefassrohren  stehenden  Fragmente  beziiglich  des  Aufbaues  ihrer  Wand 
entweder  mehr  den  Typus  der  blutleeren  oder  demjenigen  der  bluthaltigen 
Rohrchen  sich  nahern. 

Most  of  the  apparently  isolated  anlagen  seen  in  sections  are  due 
to  the  collapsing  of  the  vessels  which  connect  them  with  the  rest  of 
the  capillaries  through  the  contractility  of  the  endothelium  which 
Strieker  and  Mayer  observed.  In  sections  they  may  occur  in  great 
numbers  through  the  contraction  of  the  endothelium  due  to  the  fixing 
agent.  The  possibility  of  entirely  isolated  vessels  and  the  meaning 
of  the  apparently  isolated  vessels  are  fully  discussed  later  in  connec- 
tion with  the  theory  of  the  growth  of  lymphatic  capillaries  by  the 
addition  of  Mayer-Lewis  anlagen  in  section  VIII. 


16  Florence  R.  Sabin. 

If  it  be  established  that  the  peripheral  capillaries  grow  entirely 
from  preceding  capillaries,  then  it  is  a  perfectly  logical  hypothesis 
that  the  lymphatics  grow  from  the  veins,  from  center  to  periphery, 
rather  than  from  the  periphery  to  the  veins.  This  hypothesis  was 
advanced  by  Eanvier  (113),  based  on  the  evidence  of  the  method  of 
growth  of  the  lymphatic  capillaries  within  the  peripheral  plexus. 
Eanvier  (110-118),  in  a  series  of  studies  between  the  years  1895  and 
1807,  made  injections  of  lymphatics  in  a  variety  of  embryos.  For 
example,  he  injected  the  exceedingly  abundant  plexus  in  the  mesentery 
of  a  foetal  pig  10  cm.  long,  as  well  as  in  the  rat  and  rabbit.  He  also 
injected  the  lymphatics  in  the  amphibia.  These  injections  gave  him  an 
extensive  knowledge  of  the  form  of  the  developing  lymphatic  plexuses, 
the  valves  looking  like  collarettes  in  the  walls  of  lymphatic  ducts,  the 
irregular  capillaries  with  blunt  terminal  knobs,  of  which  especially 
large  ones  are  often  found  in  the  mesentery  as  well  as  the  slender 
sprouts,  either  ending  free  or  as  bridging  a  gap  in  the  network  to 
another  vessel.  From  all  of  these  forms  of  growing  lymphatics  Ean- 
vier saw  that  the  growth  of  lymphatics  was  from  center  to  periphery, 
and  even  though  he  thought  that  the  lymphatics  did  not  begin  in  a 
mammalian  embryo  until  it  was  about  10  cm.  long,  a  stage  at  which  the 
embryology  of  the  system  may  be  called  complete,  nevertheless  he  went 
a  step  farther  than  all  of  the  preceding  observers,  for  he  deduced  the 
hypothesis  that  the  lymphatics  grow  from  the  veins.  He  likened  their 
growth  to  the  growth  of  a  gland  (113).  In  my  own  work  I  was  led 
to  the  same  view  by  tracing  the  lymphatics  in  younger  and  younger 
embryos  back  to  the  veins,  and  the  next  section  will  include  the  evi- 
dence which  I  regard  as  essential  to  establish  the  hypothesis  of  Eanvier 
as  the  correct  theory  of  the  origin  of  the  lymphatic  system.  The  ac- 
count of  the  evidence  for  growth  by  sprouting  is,  however,  not  yet 
complete.  As  far  as  the  evidence  can  be  obtained  by  the  observation 
of  injected  specimens,  it  was  especially  well  described  and  figured 
by  AlacCallum  (79).  He  added  the  observation  of  watching  the  in- 
jection under  the  microscope  and  showed  that  extravasations  are  due 
to  explosive  rupture  of  the  lymphatic  wall. 

Injected  specimens  of  developing  lymphatic  capillaries  are  now 
well  known;  excellent  figures  are  given  by  Bartels  (10,  figs.  9  and  10, 
p.  43).  They  show  a  large  number  of  sprouts,  because  they  are  taken 
from  the  skin  of  a  pig  embryo  6.5  cm.  long.  This  is  the  exact  stage 
when  the  secondary,  superficial,  fine-meshed  plexus  is  just  growing 


The  Origin  and  Development  of  the  Lymphatic  System.       17 

from  the  deeper  coarse-meshed  or  subcutaneous  plexus,  and,  since  the 
secondary  plexus  is  growing  actively,  it  shows  many  sprouts. 

The  proof  of  growth  by  sprouting  from  injections  is  always  an 
interpretation  of  appearances,  and  though  it  is  a  logical  inference, 
one  must  turn  to  the  observations  of  the  living  form  for  conclusive 
proof.  This  has  been  given  by  E.  R.  Clark  (25-26),  who  has  again 
restudied  the  classical  object,  the  tadpole's  tail.  The  introduction 
of  chloreton  anaesthesia  is  one  of  the  factors  that  have  made  this  study 
possible,  since  the  same  specimen  can  be  kept  alive  for  weeks,  notwith- 
standing repeated  doses.  Dr.  Clark's  devise  of  an  upright  chamber, 
so  arranged  that  the  tadpole  can  be  kept  upright  without  the  pressure 
of  the  cover  slip,  and  the  finding  of  larva  of  the  form  Hyla  picker- 
ingii  in  the  spring  of  1910,  a  form  which  has  few  pigment  cells,  have 
all  combined  to  make  his  studies  so  valuable  and  convincing.  He 
has  described  not  only  the  complete  history  of  a  lymphatic  capillary, 
watched  through  periods  of  weeks,  but  has  followed  every  connective 
tissue  cell  in  the  neighborhood  (30)  through  several  generations.  Dr. 
Clark  has  shown  that  the  wall  of  the  lymphatic  capillary  is  in  ceaseless 
activity.  In  the  living  form  the  wall  is  in  part  hyaline,  in  part  granu- 
lar. The  nuclei  are  surrounded  and  obscured  by  granular  protoplasm, 
so  that  they  are  clearly  seen  only  when  dividing.  The  wall  is  of 
irregular  thickness,  often  extremely  delicate,  and  from  the  sides  and 
tips  are  sent  out  numerous  tiny  processes,  some  hyaline  and  some 
granular,  which  may  be  well  described  as  amceboid.  A  few  of  these 
persist  and  grow  into  permanent  lymphatics.  Most  of  them  are  with- 
drawn. These  tiny  sprouts,  indicating  the  functional  activity  of  the 
vessel,  do  not  disappear  in  alcoholic  specimens  (26,  p.  403).  They 
are  at  first  without  nuclei,  but  nuclei  wander  into  them  from  the 
parent  stem;  indeed,  two  nuclei  may  pass  each  other  as  they  advance 
or  recede.  This  proves  that  the  growing  amceboid  wall  of  the  lym- 
phatic capillary  is  a  syncytium  and  explains  the  failure  to  obtain  the 
silver  markings  at  the  growing  tip.  The  silver  markings  indicate 
the  more  stable  lymphatic  capillary. 

E.  E.  Clark's  most  recent  observations  (30)  show  that  increased 
activity  of  the  lymphatic  wall  in  the  sending  out  of  many  of  the  tiny 
processes  is  a  sign  of  growth,  so  that  an  area  can  thus  be  selected  to 
watch  the  processes  of  the  formation  of  new  vessels. 

As  far  as  the  relation  to  the  surrounding  connective  cells  is  con- 
cerned, the  growing,  and,  we  might  also  say,  the  functioning,  amceboid 
tips  avoid  the  cell  bodies  and  processes  of  the  mesenchyme  cells. 


18  Florence  R.  Sabin. 

Thus  there  can  be  no  question  in  regard  to  the  method  of  growth 
of  the  peripheral  capillaries.  Both  blood  and  lymphatic  capillaries 
grow  by  the  activity  of  the  protoplasm  of  their  walls.  The  endo- 
theliuin  at  the  growing  tip  is  a  syncytium,  the  processes  advance  and 
recede;  they  are  tiny,  hair-like  vessels,  and  they  receive  their  nuclei 
from  the  parent  stem.  There  are  differences  in  the  appearance  of  the 
two  types  of  capillaries  probably  associated  with  the  presence  and 
absence  of  the  circulating  blood  and  the  difference  of  function.  The 
lymphatics  have  very  many  more  tiny  sprouts,  only  a  few  of  which 
are  permanent;  that  is,  their  wall  is  much  more  amoeboid  and  the 
vessels  much  more  irregular.  Thus  on  positive  evidence,  namely,  the 
observation  of  the  process  in  the  living  form,  which  is  the  best  evi- 
dence known  to  anatomical  science,  we  are  justified  in  the  conclusion 
that  capillaries  grow  by  sprouting. 

B.    LYMPHATIC  AXD  XOX-LYMPHATIC   ZOXES. 

The  second  argument  in  favor  of  the  venous  origin  and  the  centrif- 
ugal growth  of  lymphatics  by  their  own  endothelium  is  that  it  can 
be  shown  that  lymphatic  vessels  gradually  invade  the  body  from  certain 
limited  centers.  Thus  if  injections  of  vessels  are  made  from  these 
centers  in  larger  and  larger  embryos  an  ever-increasing  zone  of 
lymphatic  capillaries  can  be  demonstrated.  Moreover,  for  each  stage 
the  zone  of  vessels  is  constant. 

I  began  my  work  with  the  study  of  the  development  of  lymphatic 
glands  in  the  embryo  pig  and  to  obtain  injections  of  them  introduced 
the  needle  into  the  foot  pads.  By  taking  younger  embryos  a  stage 
was  soon  reached  when  no  injections  of  the  foot  pads  ever  entered 
lymphatics,  but  at  these  stages  one  could  still  inject  lymphatics  in  the 
skin  over  the  body.  This  was  the  beginning  of  the  proof  that  lym- 
phatics bud  off  from  the  veins  and  grow  peripheral  ward  instead  of  the 
reverse  (Sabin  129). 

In  an  embryo  pig  o  cm.  long  (Sabin  130,  fig.  5)  a  simple  plexus  of 
lymphatic  capillaries  has  almost  entirely  covered  the  body.  I  say  capil- 
laries advisedly,  adopting  Eanvier's  (118,  p.  74)  criterion  of  valves 
for  lymphatic  ducts  in  contrast  to  their  absence  in  the  capillaries. 
The  body  is  first  invaded  by  lymphatic  capillaries,  and  the  primary 
plexus  shown  in  fig.  5  (Sabin  130),  which  is  the  anlage  of  the  deep, 
subcutaneous  plexus,  was  injected  by  a  single  puncture  of  the  needle. 


The  Origin  and  Development  of  the  Lymphatic  System.       19 

Subsequently  this  plexus  develops  valves,  but  at  this  stage  the  entire 
lymphatic  system  can  theoretically  be  injected  from  any  one  vessel, 
because  there  are  no  valves,  or,  in  other  words,  because  it  is  entirely 
a  capillary  system.  Nearly  the  same  stage  is  shown  for  a  human 
embryo  in  figs.  505  and  506  (Sabin  134).  This  was  a  remarkable  spec- 
imen, in  which  air  filled  the  lymphatics,  and  it  has  the  additional  in- 
terest of  showing  the  beginning  of  valves  in  the  vessels  of  the  skin, 
namely,  those  of  an  occipital  duct  and  in  the  axillary  and  inguinal 
ducts. 

If  earlier  stages  of  embryo  pigs  are  taken  the  cutaneous  vessels,  as 
shown  by  injecting  them,  gradually  recede,  as  seen  in  figs.  1  to  4 
(Sabin  130),  to  two  points,  one  in  the  neck  and  one  in  the  groin. 
The  earliest  lymphatics  reach  the  skin  in  the  neck  of  an  embryo  pig 
about  18  mm.  long.  All  injections  of  the  lymphatics  of  the  skin  of 
the  anterior  part  of  the  body  between  the  stages  of  2  and  5  cm.  long 
can  be  seen  to  run  into  two  symmetrical  sacs  in  the  neck  close  to  the 
jugular  vein.  These  anterior  lymph  sacs,  which  have  now  been  identi- 
fied in  a  very  great  number  of  forms,  can  be  found  in  pig  embryos  in 
earlier  stages  than  the  first  cutaneous  lymphatics.  In  my  first  paper 
I  traced  them  back  to  the  stage  of  14  mm.,  when  they  are  small  empty 
sacs,  close  against  the  anterior  cardinal  vein.  In  reality  they  begin 
in  the  pig  of  10  to  11  mm.  and  their  method  of  origin  will  be  discussed 
in  the  next  section. 

The  argument  of  lymphatic  and  non-lymphatic  zones  thus  consists 
in  the  fact  that  the  first  lymphatics  are  sacs  close  to  the  veins,  and 
from  this  stage  onward  lymphatics  can  be  injected  in  wider  and  wider 
zones  until  they  cover  the  entire  surface  of  the  body. 

It  is  necessary  to  prove  that  each  zone  represents  the  limit  of  the 
growth  of  the  lymphatics  for  a  given  stage.  To  prove  an  injection  is 
complete  for  any  stage  such  as  one  shown  in  fig.  6  (Sabin  135),  it  is 
essential  to  have  abundant  material  and  to  show  by  injection  that  the 
lymphatic  tips  always  rupture  along  a  given  line  when  the  pressure 
is  increased.  The  shape  of  the  lymphatic  vessels  at  the  line  of  growth 
will  vary  according  as  the  vessels  are  progressing  rapidly  or  not;  for 
example,  the  rounded  ends  of  fig.  2  (Sabin  129)  are  from  an  area 
where  lymphatics  grow  slowly,  while  the  margins  of  fig.  6  (Sabin  135), 
with  numerous  pointed  sprouts,  are  from  a  rapidly  growing  zone.  Fur- 
thermore, sections  of  the  skin  in  the  lymphatic  and  non-lymphatic  zones 
show  a  sharp  contrast  in  the  presence  or  absence  of  the  large  lymphatic 


20  Florence  E.  Sabin. 

capillaries,  fig.  6  (Sabin  130),  which  are  easy  to  identify.  The  method 
of  sections  is,  however,  inadequate  to  define  the  limits  of  growth, 
except  for  the  large  vessels,  for  the  tiny  lymphatic  sprouts  can  only 
be  found  in  sections  when  the  plane  of  cutting  is  favorable. 

This  progression  of  the  lymphatics  in  the  skin  has  been  confirmed 
by  Polinski  (107)  in  bovine  embryos.  The  patterns  which  he  shows 
make  a  most  interesting  comparison  with  those  of  pig,  cat  and  human 
embryos.  In  the  bird  the  same  progression  of  zones  is  shown  to  some 
extent  in  the  injections  of  Mierzejewski  (96). 

Beside  the  definite  zones  the  lymphatics  grow  into  definite  layers. 
Banvier  (118)  noted  that  the  superficial  lymphatics  are  deeper  than  the 
blood  vessels.  They  follow  the  vessels  into  the  skin  and  each  plexus  is 
deeper  than  the  corresponding  blood  vascular  plexus1  ( Sabin  130) .  The 
development  of  the  lymphatics  into  the  intestine,  as  worked  out  by 
Heuer  (43),  illustrates  the  point  of  the  definite  layers  into  which  the 
lymphatics  grow  particularly  well.  The  lymphatics  for  the  intestine 
grow  from  the  retroperitoneal  sac,  which  arises  from  the  renal  veins 
(Baetjer,  8).  The  vessels  form  a  rich  plexus  in  the  mesentery  and 
from  this  plexus  a  series  of  vessels  grow  into  the  intestinal  wall  and 
penetrate  to  the  submucosa.  Here  they  form  at  first  a  series  of  units 
shown  by  Heuer  in  fig.  10  (43),  which  has  been  copied  as  fig.  508 
(Sabin  134).  These  primary  vessels  soon  form  the  submucosal  plexus 
of  ducts,  from  which  the  mucosal  capillary  plexus  and  lacteals  develop 
on  the  one  hand  and  the  serosal  vessels  on  the  other. 

This  fact,  that  the  body  is  invaded  by  lymphatics,  has  an  especial 
interest,  because  it  was  the  clue  which  enabled  us  to  trace  back  the 
systems  of  lymphatic  ducts  to  their  beginnings  and  show  that  they 
arise  from  the  veins  and  grow  toward  the  periphery.  It  is,  further- 
more, the  key  by  which  the  development  of  the  lymphatic  system 
within  each  organ  may  be  traced  to  the. ultimate  capillaries.  It  is 
considered  again  in  relation  to  the  development  of  lymph  sacs,  for 
there  it  can  be  taken  up  in  connection  with  its  exact  relations  to 
vascular  and  non-vascular  zones.  Both  the  blood  vascular  and  the 
lymphatic  systems  invade  the  body  by  the  growth  of  their  capillaries. 
Each  system  has  its  definite  zones  and  layers  which  can  be  followed  in 
orderly  progression.  The  invasion  of  the  body  by  the  blood  capillaries 
is  more  complete  than  by  the  lymphatic  capillaries;  for  example,  the 
lymphatic  capillaries  do  not  grow  into  the  central  nervous  system,  nor 
into  the  interspaces  between  muscle  fibers. 


The  Origin  and  Development  of  the  Lymphatic  System.       21 

C.    DIRECT  BUDDING  OF  THE  LYMPHATICS  FROM  THE  VEINS. 

As  has  been  stated,  it  was  the  study  of  the  decreasing  zones  of  lym- 
phatic capillaries  which  led  to  the  discovery  that  the  first  lymphatics 
are  sacs  close  to  the  veins.  In  mammalian  forms  the  first  lymphatics 
are  the  anterior  lymph  sacs  which  bud  off  from  the  anterior  cardinal 
veins.  These  jugular  lymph  sacs  were  noted  by  Saxer  (138)  in  1896 
in  his  extensive  study  on  lymph  glands,  but  he  did  not  recognize 
their  significance.  They  can  readily  be  made  out  in  sections  of  pig 
embryos  14-20  mm.  long,  or  in  sections  of  any  mammalian  embryo 
of  the  corresponding  stages.  The  fact  that  they  are  the  first  lymphatics 
in  mammals  and  that  they  lie  close  to  the  vein,  separated  only  by  the 
double  endothelial  wall,  led  me  to  the  conclusion  that  the  lymphatics 
budded  directly  off  from  the  veins  and  it  is  gratifying  to  find  that 
this  reasoning  has  been  justified  by  further  investigation  (Sabin  129, 
E.  E.  &  E.  L.  Clark  29  and  Hoyer  49a) . 

To  follow  the  development  of  our  knowledge  in  historical  sequence 
the  next  step  was  taken  by  F.  T.  Lewis  (76)  in  a  series  of  excellent 
reconstructions  of  the  anterior  lymph  sacs  in  rabbit  embryos,  the  best 
figures  we  have  had  up  to  the  present  time  of  the  form  of  the  early 
sac.  Lewis  carried  the  study  still  further  back  and  showed  that  the 
definite  sac  was  preceded  by  a  blood-filled  capillary  plexus.  This  blood- 
filled  plexus  has  proved  to  be  a  very  important  observation.  From  the 
presence  of  blood  Lewis  made  the  logical  inference,  with  the  evidence 
at  his  command,  that  these  vessels  were  blood  capillaries  and  that 
hence  lymphatics  were  transformed  blood  capillaries.  The  observation 
of  the  blood-filled  vessels  has  been  confirmed  by  Huntington  and 
McClure  (54)  on  the  cat  and  by  myself  in  human  and  pig  embryos 
(132-133)  and  we  have  all  accepted  the  theory  of  transformed  capil- 
laries until  new  evidence  has  now  shown  us  the  correct  explanation 
of  the  presence  of  the  blood. 

This  new  evidence  has  been  furnished  by  the  work  of  E.  E.  and 
E.  L.  Clark  (27-29).  E.  L.  Clark  began  with  the  study  of  the  allan- 
toic  lymphatic  vessels  in  chick  embryos.  From  the  allantoic  vessels 
she  was  led  back  to  the  posterior  lymph  hearts  and  the  lymphatics  of 
the  body  wall.  The  injections  of  Mierzejewski  (96)  show  that  the 
primary  line  of  growth  for  the  lymphatic  vessels  in  the  skin  of  the 
chick  is  along  the  thoraco-epigastric  vein  (96,  figs.  1  to  4).  E.  L. 
Clark  noted  that  in  early  stages  the  lymphatic  vessels  along  this  line 


22  Florence  R.  Sabin. 

were  filled  with  blood  and  that  the  blood  in  them  was  stagnant,  in 
striking  contrast  to  the  rapidly  circulating  venous  blood.  The  fact 
that  the  lymphatics  invade  the  skin  in  the  chick  while  they  are  in  the 
blood-filled  stage  gives  at  once  an  advantage.  By  means  of  fine  glass 
cannulas  E.  .L.  Clark  was  able  to  inject  individual  vessels  of  this  blood- 
filled  plexus  in  the  living  chick  and  watch  the  ink  flow  to  the  pul- 
sating lymph  heart  and  thence  to  the  veins.  These  blood-filled  vessels 
have  the  usual  characteristics  of  lymphatic  capillaries,  in  that  they 
are  larger  and  more  irregular  than  blood  capillaries,  but  the  essential 
point  in  E.  L.  Clark's  work  is  that  by  using  a  form  in  which  the  lym- 
phatics could  be  seen  in  the  living  embryos  she  discovered  a  new 
criterion  for  recognizing  lymphatics,  namely,  the  ^stagnant  blood  in 
contrast  to  the  circulating  blood. 

With  this  important  criterion  she  has  followed  back  the  origin  of 
the  posterior  lymph  hearts.  Since  Sala  (137)  it  has  been  known  that 
in  the  chick  the  posterior  lymph  hearts  develop  opposite  the  lateral 
branches  of  the  first  five  coccygeal  veins  (137,  p.  269).  Sala  described 
the  process  as  beginning  during  the  sixth  day  by  a  hollowing  out  of 
spaces  in  the  mesenchyme,  but  by  watching  the  living  chick  embryo 
during  the  fifth  day  E.  E.  and  E.  L.  Clark  have  seen  that  the  skin 
over  the  first  five  coccygeal  veins  is  a  comparatively  non-vascular  zone, 
so  that  brilliant  direct  illumination  enables  them  to  see  in  the  depth 
a  series  of  tiny  blood-filled  buds  close  to  the  main  coccygeal  vein  and 
its  branches.  The  blood  in  these  buds  is  of  slightly  different  color 
from  the  circulating  blood.  They  proved  that  these  buds  are  always  in 
connection  with  the  parent  vein,  for  a  direct  injection  of  them  always 
runs  over  into  the  vein,  but  no  injection  of  the  peripheral  blood  capil- 
laries ever  fills  them.  Hence  they  bud  off  from  the  veins  and  are  filled 
with  blood  from  a  back-flow  from  the  parent  vein.  It  is  of  course  evi- 
dent that  this  process  cannot  be  seen  in  the  body  wall  of  the  chick  with 
the  clearness  with  which  every  cell  division  can  be  followed  in  the 
tadpole's  tail;  nevertheless,  the  stagnant  blood  has  been  proved  by  injec- 
tion to  practically  fill  the  developing  vessels,  so  that  it  is  an  adequate 
criterion  of  the  extent  of  their  development. 

By  selecting  a  chick  which  shows  these  primary  lymphatic  buds  and 
keeping  it  under  observation  under  high  power  in  a  warm  chamber 
E.  R.  and  E.  L.  Clark  (29)  have  been  able  to  watch  these  blood-filled 
lymphatic  buds  join  with  each  other  to  form  a  deep  circumscribed 
lymphatic  plexus,  the  anlage  of  the  posterior  lymph  heart.  It  is  clear 


The  Origin  and  Development  of  the  Lymphatic  System.       23 

from  Sala's  description  that  it  was  this  plexus  which  he  described 
as  the  beginning  of  the  lymphatics.  While  the  deep  lymphatic  capil- 
lary plexus  is  being  transformed  into  a  lymph  sac  and  pulsating 
lymph  heart,  lymphatic  vessels  are  already  growing  toward  the  skin, 
where  they  gradually  extend.  They  grow  first  along  the  lateral  line 
to  anastomose  with  the  jugular  lymphatics.  There  is  a  continuous 
peripheral  growth  of  the  lymphatics  from  the  time  of  the  first  buds 
on,  and  the  entire  blood-filled  stage  takes  but  a  few  hours.  Neverthe- 
less, it  is  long  enough  to  enable  one  to  watch  the  buds  form  a  plexus ; 
the  plexus  becomes  a  sac  and  lymph  heart,  and  the  lymphatics  from  it 
spread  out  in  the  skin.  Injections  prove  that  in  every  stage  of  their 
development,  from  the  primary  buds  onward,  the  lymphatics  connect 
with  the  parent  veins;  thus  there  is  no  separation  from  the  vein  and 
subsequent  rejoining,  as  all  of  the  recent  workers,  Lewis,  Huntington 
and  McClure  and  myself,  have  thought  from  the  difficulty  of  finding 
the  connections  with  the  veins  in  serial  sections. 

The  posterior  lymph  heart  in  the  chick  is  especially  advantageous 
for  study,  since  it  is  in  the  first  place  sufficiently  superficial  to  be 
seen  in  the  living  embryo,  and  in  the  second  place  it  is  covered  by  a 
non -vascular  zone.  In  mammalian  embryos  no  caudal  lymph  hearts 
develop.  It  is  very  interesting  to  note  in  injected  embryo  pigs  that 
the  corresponding  area  in  the  skin  at  the  root  of  the  tail,  instead  of 
being  a  non-vascular  zone,  is  an  area  of  especially  wide  blood  capil- 
laries. This  area,  seen  in  fig.  467  (Evans  35),  is  constant,  and,  as 
it  almost  always  retains  a  little  blood,  can  be  injected  when  a  partial 
injection  of  the  veins  of  the  lower  part  of  the  body  is  desired. 

The  work  of  the  Clarks  adds  the  final  proof  of  the  theory  that  the 
lymphatics  bud  off  from  the  veins.  Moreover  their  method  is  so  simple 
that  anyone  may  convince  himself  of  the  essential  results  by  a  few 
hours  observations.  Their  injections  have  corrected  two  errors, 
namely,  the  idea  that  the  lymphatics  are  transformed  blood  capillaries 
and  that  they  temporarily  separate  from  the  veins. 

Thus  I  regard  as  proved  the  three  points  which  have  been  stated 
as  essential  in  establishing  the  origin  of  the  lymphatics  from  the 
veins.  Stated  in  reverse  order  they  are :  First,  the  lymphatic  system 
begins  as  a  series  of  endothelial  buds  which  sprout  out  directly  from 
the  veins  as  a  new  type  of  vessels,  namely,  lymphatics.  These  buds 
unite  to  form  plexuses  which  develop  into  sacs.  These  sacs  may  become 
lymph  hearts.  Second,  from  these  lymph  sacs  or  hearts  lymphatic 
2 


24  Florence  E.  Sabin. 

capillaries  gradually  invade  the  body  in  orderly  sequence  in  definite 
and  characteristic  zones  and  layers.  The  growth  is  always  in  the 
capillary  bed,  that  is,  all  lymphatics  develop  as  capillaries,  and  the 
earliest  ones  to  develop  become  the  first  lymph  trunks  or  ducts.  Third, 
the  method  of  growth  throughout  is  by  the  sprouting  of  the  proto- 
plasm and  the  nuclear  division  of  the  endothelial  of  the  capillary  wall. 
The  ultimate  capillaries  are  distinguished  by  the  absence  of  valves. 
In  a  word,  there  is  a  continuous  growth  and  invasion  of  the  body  by 
lymphatic  capillaries  from  the  primary 'sprouts  which  bud  off  from 
the  veins  to  the  ultimate  peripheral  lymphatic  capillaries. 

IT.  PRIMAEY  LYMPHATIC  SYSTEM  IX  ^)IFFEREXT 
FOEMS. 

1.  PRIMARY  LYMPHATIC  SACS  ix  THE  PIG. 

A.    GENERAL  SUMMARY  OF  THE  SACS. 

I  shall  begin  the  section  on  the  special  anatomy  of  the  lymph  sacs 
by  a  detailed  description  of  the  origin  of  the  lymphatics  in  the  pig, 
since  that  is  the  best  known  form.  I  can  now  describe  the  origin  of 
all  of  the  lymphatic  sacs  in  the  pig  and  give  the  relation  of  the  main 
ducts  to  all  of  these  primitive  sacs.  On  the  basis  of  our  knowledge  of 
the  lymphatic  system  in  this  form  I  shall  give  what  is  known  of  the 
lymphatic  system  in  other  mammals,  in  birds,  reptiles  and  amphibia, 
and  shall  then  be  in  a  position  to  compare  the  primary  lymph  sacs  in 
mammals  with  the  amphibian  lymph  hearts  and  sacs. 

In  the  pig  the  lymphatics  bud  off  from  the  veins  in  two  places,  from 
the  anterior  cardinal  veins  and  from  the  veins  of  the  Wolffian  body. 
There  are  two  sets  of  paired  sacs,  the  jugular  and  the  iliac;  and  two 
unpaired  sacs,  the  retroperitoneal  and  the  cisterna  chyli.  In  the  most 
general  terms  the  jugular  sacs  drain  the  anterior  half  of  the  body; 
the  iliac  sacs  drain  the  posterior  half  of  the  body,  while  the  retroperi- 
toneal or  prse-aortic  sac  drains  the  viscera.  The  cisterna  chyli  with 
the  thoracic  duct  connects  the  jugular  and  renal  lymphatics.  In  more 
specific  terms  the  jugular  lymphatics  bud  off  from  the  anterior  cardi- 
nal veins,  form  large  sacs  in  the  neck,  from  which  lymphatics  grow 
to  the  head  and  neck,  the  foreleg  and  thorax,  and  to  the  heart  and 
lungs.  From  the  left  sac  arises  the  jugular  stem  of  the  thoracic  duct. 
All  the  rest  of  the  lymphatic  sacs  bud  off  from  the  vena  cava,  where  it 
is  a  part  of  the  median  mesonephritic  vein,  from  the  median  mesone- 


The  Origin  and  Development  of  the  Lymphatic  System.       25 

phritic  vein  and  the  veins  in  the  dorso-medial  edge  of  the  Wolffian 
bodies.  The  lymphatics  which  grow  from  the  ventral  surface  of  the 
mesonephritic  veins  make  the  large  retroperitoneal  or  prae-aortic  sac; 
those  which  grow  along  the  dorso-medial  edge  of  the  Wolffian  bodies, 
lateral  to  the  aorta,  form  the  iliac  sacs,  and  those  which  grow  dorsal 
to  the  aorta  make  the  cisterna  chyli  and  thoracic  duct.  The  thoracic 
duct  is  formed  in  small  part  from  a  duct  from  the  left  jugular  sac; 
in  larger  part  from  a  plexus  which  surrounds  the  aorta  from  the 
cisterna  chyli.  The  retroperitoneal  sac  drains  the  diaphragm  and  the 
abdominal  viscera,  except  the  permanent  kidney;  the  iliac  sacs  drain 
the  permanent  kidney,  the  abdominal  walls,  the  hindlegs,  tail  and  em- 
bryonic membranes. 

B.    THE   JUGULAR   SACS. 

As  has  already  been  stated  in  part  III,  the  spreading  of  the  primary 
superficial  lymphatic  capillaries  in  the  skin  of  the  embryo  pig  takes 
place  while  the  embryo  is  growing  from  20  to  50  mm.  long.  The  first 
lymphatic  buds  are  found  when  the  embryo  is  about  11  mm.  long 
(fig.  6) ;  the  sac  is  well  formed  at  18  mm.  (fig.  7),  and  reaches  its 
maximum  size  in  an  embryo  30  mm.  long  (fig.  8).  It  remains  as  a 
large  sac  until  the  embryo  measures  50  mm.,  when  it  begins  to  be 
transformed  in  part  into  lymph  glands  (fig.  16). 

An  understanding  of  the  development  of  the  jugular  sac  depends 
on  a  knowledge  of  the  veins  of  the  region.  Indeed,  I  was  nqt  able  to 
inject  the  early  stages  until  I  had  so  mastered  the  pattern  of  the 
blood  vessels  that  a  little  blood  in  any  of  them  enabled  me  to  repro- 
duce the  whole  picture  and  so  avoid  the  blood  vessels. 

The  study  of  the  form  of  the  early  anterior  cardinal  vein  is  shown 
for  the  chick  by  Evans  (34)  in  his  figs.  1  to  3.  These  figures  show 
that  the  primitive  deep  dorsal  segmental  branches  drain  the  ventro- 
lateral  surface  of  the  spinal  cord.  They  bring  out  the  gradual  curve 
of  the  anterior  cardinal  vein  and  the  more  marked  bend  of  the  pos- 
terior cardinal  vein,  where  the  two  join  the  duct  of  Cuvier.  This 
stage  is  shown  for  the  chick  in  cross  section  by  Evans  (35)  in  his 
fig.  43?  B,  as  well  as  in  (Evans  34)  his  fig.  4. 

In  fig.  4  is  given  an  injection  of  the  blood  vessels  in  an  embryo  pig 
7  mm.  long,  a  stage  just  before  the  lymphatics  begin.  The  curve  of 
the  dorsal  border  of  the  cardinal  veins  toward  the  duct  of  Cuvier  is 


26 


Florence  R.  Sabin. 


slight,  not  as  marked  as  in  the  chick,  nor  as  it  is  on  the  left  side  of  this 
same  embryo. 

Arising  from  the  cardinal  veins  there  is  a  series  of  seven  deep 
segmental  branches  extending  to  the  nervous  system  between  the 
area  of  the  primitive  posterior  cerebral  vein  (Mall  143),  which  arches 


FIG.  4. — Injected  specimen  to  show  the  anterior  cardinal  vein  and  its 
branches  ir  an  embryo  pig  which  measured  7  mm.  in  its  greatest  diameter. 
Magnified  ;  bout  28  times.  This  is  a  stage  before  the  lymphatic  vessels 
have  begun.  A  =  aorta  arches;  D.  C.  =  duct  of  Cuvier;  V.  c.  p.  =  vena 
cerebralis  posterior;  V.  5  =  fifth  segmental  vein. 

downward  over  the  medulla  and  cord  and  the  cerebral  end  of  the  arm 
bud.  The  first  two  of  these  branches  drain  the  medulla,  and  hence 
there  are  five  spinal  or  segmental  branches  proper.  Each  of  these 
branches  has  a  corresponding  artery  which  lies  slightly  caudalward 
and  nearer  the  midline.  Only  the  lower  four  segmental  arteries  are 
injected  in  this  specimen.  The  relative  position  of  artery  and  vein 


The  Origin  and  Development  of  the  Lymphatic  System.       27 

shows  in  fig.  43 7  (Evans  35).  Each  of  the  five  spinal  veins  drains 
a  plexus  of  wide  capillaries,  which  marks  the  presence  of  the  develop- 
ing spinal  ganglia. 

It  is,  however,  the  superficial  veins  which  are  of  more  interest  from 
the  standpoint  of  the  lymphatic  system.  Along  the  dorso-lateral  bor- 
der of  the  cardinal  veins  there  is  a  series  of  intersegmental  sprouts, 
varying  from  one  to  four  or  five  in  each  interspace.  These  sprouts 
drain  a  superficial  capillary  plexus  which  is  developing  in  a  groove 
in  the  neck  which  is  a  continuation  of  the  groove  dorsal  to  the  Wolffian 
ridge.  The  arteries  for  this  superficial  capillary  plexus  come  from  the 
deep  segmental  arteries  (Evans  35,  fig.  437).  The  connections  of 
the  plexus  with  the  cardinal  vein  are  most  abundant  near  the  poste- 
rior cerebral  vein.  This  superficial  capillary  plexus  of  the  body  wall 
has  a  most  interesting  development.  For  the  area  between  the  limb 
buds  it  has  been  worked  out  by  Dr.  Helen  W.  Smith  (141),  whose 
figures  are  copied  by  Evans  (35).  This  superficial  plexus  in  the  tail 
region  shows  a  constant  zone  of  widened  capillaries  (35,  fig.  467) 
corresponding  to  the  position  of  the  posterior  lymph  hearts  in  birds. 
For  the  neck  region  it  is  necessary  to  know  this  plexus  in  order  to 
locate  the  jugular  lymphatics,  and  therefore  its  development  will  be 
followed  in  further  stages. 

The  ventral  branches  of  the  anterior  cardinal  vein  are  represented 
at  this  stage  by  one  large  pericardinal  vein,  which  receives  branches 
from  the  gill  arch  region.  The  common  stem  for  the  branchial  and 
pericardial  vessels  connects  with  the  anterior  cardinal  vein,  but  it 
drains  directly  into  the  duct  of  Cuvier  (see  Grosser  and  F.  T.  Lewis 
as  quoted  by  Evans  35,  p.  660)  on  the  right  side  of  this  embryo, 
while  on  the  left  side  of  the  same  specimen  it  opens  into  the  anterior 
cardinal  vein. 

The  earliest  lymphatic  buds  have  been  found  in  embryo  pigs 
measuring  from  10  to  11  mm.  long.  All  of  my  measurements  mean 
the  greatest  length  and  are  made  on  the  fresh  specimen.  The  stage 
of  the  first  lymphatics  is  shown  in  two  figures,  fig.  5,  a  total  mount 
of  an  embryo  injected  with  India  ink  through  the  umbilical  artery 
in  a  specimen  11  mm.  long,  and  fig.  6,  a  cross  section  250  ^  thick, 
through  the  lymphatic  area  in  an  embryo  measuring  11  mm.  The 
blood  vessels  were  injected  with  India  ink,  and  the  lymphatics  are  filled 
with  blood. 


Florence  R.  Salin. 


V.c.p. 


FIG.  5. — Injected  specimen  of  the  anterior  cardinal  \ein  and  its  branches 
in  an  embryo  pig  measuring  11.  mm.  in  its  greatest  diameter.  Magnified 
about  33  times.  This  is  the  stage  at  which  the  lymphatic  vessels  begin  to 
develop.  The  vessels  were  injected  with  India  ink  through  the  umbilical 
artery.  U.  C.  =  duct  of  Cuvier;  V.  c.  p.  =  vena  cerebralis  posterior;  P.  u.  v. 
—  primitive  vena  ulnaris;  V.  5  =  fifth  segmental  vein. 


The  Origin  and  Development  of  the  Lymphatic  System.       29 

The  blood  vessels  show  a  great  development.  The  five  spinal  seg- 
mental  veins  are  clear,  draining  the  large  plexuses  around  each  spinal 
ganglion;  the  ganglia  are  now  farther  ventralward  than  in  the  pre- 
ceding stage.  The  superficial  capillary  plexus  shows  the  most  marked 
changes.  There  is  a  continuous  sheet  of  superficial  capillaries  in  the 


9— 


L.S-- 


FIG.  6. — Section  through  the  third  segmental  vein  in  an  injected  embryo 
pig  which  measured  11  mm.  in  its  greatest  diameter.  The  section,  which  is 
250  /j.  thick,  shows  the  relation  of  the  jugular  lymphatic  sprouts  or  buds 
to  an  anterior  cardinal  vein.  Magnified  about  40  times.  The  blood  vessels 
were  injected  with  India  ink  through  the  umbilical  artery.  The  lymphatic 
sprouts,  L.  s.,  have  a  natural  injection  of  blood;  these  lymphatic  sprouts  lie 
in  a  non-blood-vascular  zone  dorso-lateral  to  the  anterior  cardinal  vein. 
A.  c.  v.  —  vena  cardinalis  anterior;  g  —  groove. 

neck,  anastomising  with  the  branchial  vessels,  with  the  pericardial 
vessels  and  with  the  abundant  capillary  plexus  of  the  arm  bud. 
Opposite  the  medulla  the  connections  of  this  superficial  plexus  with 
the  anterior  cardinal  vein  are  very  numerous,  while  opposite  the  heart 
a  few  veins  connect  it  either  directly  with  the  anterior  cardinal  vein 
or  more  frequently  with  the  main  segmental  branches.  It  is  this 


30  Florence  R.  Sabin. 

zone,  opposite  the  third,  fourth  and  fifth  segmental  veins,  that  is  the 
lymphatic  area. 

To  locate  the  lymphatic  area  definitely  it  is  necessary  to  note  the 
details  of  the  superficial  veins  more  carefully.  The  superficial  plexus 
along  the  lateral  groove  is  finer  meshed  and  of  smaller  capillaries  than 
the  rest.  The  groove  is  an  important  land  mark  and  it  shows  best  in 
the  cross  section,  fig.  6.  From  the  plexus  in  the  groove  a  sheet  of  wide 
capillaries  is  growing  dorsalward  external  to  the  myotomes,  and  loops 
from  this  sheet  connect  with  the  deep  plexus  around  the  spinal  cord. 
The  skin  over  the  entire  dorsal  surface  of  the  cord  is  an  entirely  non- 
vascular  area  at  this  stage.  From  the  ventral  border  of  the  super- 
ficial plexus  in  the  groove  a  sheet  of  wider  and  more  open-meshed  capil- 
laries covers  the  surface  of  the  embryo  over  the  anterior  cardinal 
vein.  The  surface  contour  of  this,  the  lymphatic  area,  is  a  swelling 
seen  in  fig.  6,  which  I  shall  call  the  jugular  lymphatic  ridge.  The 
blood  capillaries  of  this  area  are  very  superficial  and  drain  into  the 
cardinal  vein  in  four  ways :  (1)  Through  the  plexus  in  the  groove  and 
its  dorso-lateral  superficial  veins:  (2)  in  small  part  through  direct 
branches  to  the  lateral  surface  of  the  cardinal  vein;  (3)  through 
branches  of  the  ventro-lateral  surface  of  the  anterior  cardinal  vein 
which  drain  the  pericardium  (fig.  5) ;  and  (4)  through  anastomoses 
with  the  capillaries  (fig.  5)  of  the  primitive  ulnar  vein.  The  primi- 
tive ulnar  vein  has  developed  from  the  diffuse  capillary  arm  bud 
plexus  of  the  preceding  stage.  It  enters  the  lateral  surface  of  the 
posterior  cardinal  vein  directly  opposite  the  fifth  segmental  branch. 
Thus  a  part  of  the  blood  of  the  branchial  region  and  of  the  pericar- 
dium drains  through  the  superficial  plexus  over  the  lymphatic  ridge 
into  the  primitive  ulnar  vein.  In  a  little  older  embryos,  namely,  those 
12  mm.  long,  a  second  vein  has  developed  from  the  arm  bud  plexus 
just  cerebralward  to  the  primitive  ulnar,  which  now  receives  the  blood 
from  the  plexus  of  the  lymphatic  ridge.  The  complete  description  of 
this  superficial  plexus  involves  a  study  of  the  origin  of  the  external 
jugular  vein  which  I  do  not  wish  to  go  into  until  I  can  illustrate  it 
adequately.  I  bring  up  the  point  here  because  A.  H.  Clark  (24)  has 
shown  that  occasionally  a  lymph  trunk  in  the  neck  in  older  pigs  opens 
directly  into  the  external  jugular  vein  near  its  point  of  union  with  the 
internal  jugular  vein.  It  is  therefore  of  importance  to  note  that  the 
capillary  plexus  in  the  arm  bud,  from  which  this  part  of  the  internal 
jugular  vein  comes,  is  present  in  the  embryo  when  the  lymphatics  are 


Tlie  Origin  and  Development  of  the  Lymphatic  System.       31 

budding  out,  though  the  internal  jugular  vein  as  a  whole  is  formed 
considerably  later. 

For  the  present  the  interest  in  the  superficial  blood  capillary  plexus 
is  that  it  covers  the  lymphatic  ridge.  It  will  be  seen  in  fig.  6  that  the 
lymphatic  ridge  includes  a  non-vascular  area.  This  non-vascular  area 
is  bounded  by  the  anterior  cardinal  vein  and  its  dorsal  branches,  the 
plexus  of  the  groove,  the  wide-meshed  superficial  plexus  and  the 
ventro-lateral  branches  of  the  anterior  cardinal  vein.  An  occasional 
direct  lateral  branch  of  the  cardinal  vein  cuts  through  the  otherwise 
non-vascular  area.  Within  this  area  are  seen  the  blood-filled  lymphatic 
buds.  They  lie  in  the  angle  between  the  anterior  cardinal  vein  and  its 
dorso-lateral  branches  and  connect  with  both.  They  are  opposite  the 
third,  fourth  and  fifth  dorsal  segmental  veins,  and  extend  a  distance 
of  1.5  mm.  from  the  primitive  ulnar  along  the  anterior  cardinal  vein. 
The  tiny  lymphatic  buds  are  already  sending  sprouts  away  from  the 
parent  vein. 

The  presence  of  stagnant  blood,  which  has  proved  so  valuable  a 
criterion  in  the  living  embryo,  must  be  used  guardedly  in  studying 
sections,  unless  there  is  a  complete  vascular  injection.  In  the  lym- 
phatic area  I  have  noted  in  the  fresh  embryo  that  the  blood  capillary 
plexus  of  the  groove  is  often  empty,  while  the  superficial  plexus  over 
the  lymphatic  area  and  over  the  pericardium  tend  to  retain  some 
blood.  The  blood  vessels  of  an  area  where  lymphatics  are  budding 
must  be  thoroughly  known  before  one  can  be  sure  of  the  lymphatic 
buds.  The  early  lymphatic  buds  are  packed  with  blood  to  an  extent 
not  common  for  the  veins,  which  perhaps  shows  best  in  figs.  490  and 
491  (Sabin  134),  for  the  jugular  lymphatic  buds  in  a  human  embryo. 

I  have  not  yet  succeeded  in  injecting  the  first  jugular  lymphatic 
buds  in  the  pig  as  E.  L.  Clark  has  done  for  the  early  stages  of  the 
posterior  lymph  heart  of  the  chick.  In  the  embryo  12  mm.  long  they 
can  IDC  seen  in  the  fresh  embryo,  and  sections  show  that  they  have 
formed  a  plexus  along  the  margin  of  the  anterior  cardinal  vein.  To 
see  the  lymphatic  buds  at  12  mm.  the  embryo  should  be  placed  in  warm 
Locke's  solution  while  the  heart  is -still  beating,  and  it  should  be  viewed 
under  the  high  powers  of  the  binocular  microscope  with  the  direct 
sunlight  focused  upon  it.  The  lymphatic  area  is  wider  in  the  dorso- 
ventral  direction  than  in  fig.  6,  but  its  dorsal  boundary  is  clearly 
marked  by  the  surface  groove.  If  the  superficial  blood  capillary 
plexus  is  empty  the  lymphatics  can  then  be  seen  looking  like  a  cluster 


Florence  E.  Sabin. 


of  dull  red  grapes  against  the  cardinal  vein;  in  the  older  specimens 
some  of  the  buds  are  markedly  larger  than  others,  while  at  14  mm. 
the  largest  of  them  makes  a  definite  sac  with  large  sprouts  projecting 
dorsalward.  At  this  stage,  namely,  14  mm.,  the  sac  can  be  injected 


FIG.  7. — Injection  of  the  left  jugular  lymph  sac  in  an  embryo  pig  which 
measured  18  mm.  in  its  greatest  diameter.  The  specimen  was  fixed  in  10 
per  cent  formol  and  cleared  by  the  Spalteholz  method.  Magnified  about  30 
times.  The  glass  canula  which  shows  piercing  the  upper  border  of  the  arm 
bud  shows  the  point  of  injection.  A.  c.  —  anterior  curvature  of  the  lymph 
sac;  A.  s.  =  apex  of  the  lymph  sac;  I.  =  glass  canula  used  for  injection; 
S.  s.  =  stalk  of  the  lymph  sac;  V.  j.  i.  =  vena  jugular  interna. 

by  direct  puncture.  The  preceding  or  plexus  stage  is  shown  by  E.  L. 
Clark  for  the  corresponding  sac  in  the  chick  in  fig.  15.  The  beginning 
sac  is  shown  for  the  human  embryo  in  fig.  491  (Sabin  134). 

It  is  not  easy  to  get  perfect  injections  by  direct  puncture,  but  by 
the  time  the  embryo  is  10  mm.  the  dorsal  sprouts  from  the  sac  are 


The  Origin  and  Development  of  Hie  Lymphatic  System.       !>3 


S.s.. 


FIG.  8.— Injection  of  the  right  jugular  lymph  sac  in  foetal  pig  measuring 
3.5  cm.  Magnified  about  10  times.  After  A.  H.  Clark  (24) ;  this  specimen 
was  shown  as  fig.  3  in  130;  it  has  since  been  cleared  by  the  Spalteholz 
method  so  that  it  shows  the  relation  of  the  superficial  lymph  vessels  to  the 
jugular  sac.  It  shows  a  complete  injection  of  the  suprascapular  and  oc- 
cipital plexuses,  but  an  incomplete  injection  of  the  superficial  cervical 
plexus.  S.  s.  =  the  stalk  of  the  sac  showing  faintly  through  the  shoulder; 
V.  1.  f.  =  vena  linguofacialis. 


34  Florence  E.  Sabin. 

sufficiently  large  so  that  the  sac  can  be  injected  indirectly  through 
them. 

The  process  by  which  a  lymphatic  plexus  becomes  a  sac  has  been 
called  confluence  or  cavernization  by  Eanvier  (117-118).  He  says 
that  when  two  lymph  vessels  lie  in  contact  the  endothelial  wall  between 
them  disappears  (possibly  it  is  retracted)  and  thus  large  sacs  develop. 
It  is  exceedingly  interesting  that  this  process  does  not  take  place  when 
lymphatic  endothelium  rests  on  venous  endothelium,  as  is  the  case 
of  the  jugular  lymph  sac. 

The  most  interesting  stage  in  the  formation  of  the  jugular  sac  is 
shown  in  fig.  7  from  an  embryo  18  mm.  long.  To  inject  the  sac  at 
this  stage  it  is  again  important  to  note  the  plan  of  tlje  blood  capillaries 
In  fig.  6  it  will  be  noted  that  there  is  a  tiny  blood  capillary  which  cuts 
across  the  lymphatic  area  from  the  superficial  plexus  to  the  deeper 
veins  and  divides  the  lymphatic  area  into  two  parts,  a  ventral  jugular 
part  and  a  dorsal  part.  By  the  time  the  embryo  is  from  16-20  mm. 
long  the  path  of  this  slender  vessel  is  occupied  by  a  considerable  plexus 
of  blood  vessels  and  by  nerves  as  well.  The  sprouts  from  the  main 
jugular  sac  (fig.  7)  have  grown  up  into  the  dorsal  lymphatic  area  and 
there  developed  into  an  abundant  plexus.  It  is  this  plexus  which  can 
be  injected,  as  is  shown  by  the  glass  tube  which  pierces  the  arm  bud. 
The  original  sac  is  shown  just  lateral  to  the  shadow  of  the  internal 
jugular  vein.  This  is  difficult  to  inject  by  direct  puncture,  because 
the  blood  capillary  plexus  over  it  has  become  double  and  is  excessively 
abundant.  Moreover  the  sac  is  now  just  mesial  to  the  developing 
external  jugular  vein  and  lies  very  close  to  it.  The  blood  capillary 
plexus  of  the  groove  shown  in  fig.  6  is  now  deeper  in,  and  the  entire 
area  of  the  groove  so  dense  a  vascular  area  that  no  canula  can  enter  the 
zone  without  filling  the  blood  capillaries.  But  just  ventral  to  the 
groove  in  the  dorsal  lymphatic  area  almost  every  puncture  will  fill  the 
lymphatics,  provided  it  avoids  the  superficial  blood  capillaries.  This 
dorsal  lymphatic  area  is  destined  to  be  the  posterior  triangle  of  the 
neck;  even  in  this  early  stage  it  looks  translucent  and  continues  to  do 
so,  for  it  is  always  uncovered  by  muscle,  lying  between  the  trapezius 
and  the  sternocleidomastoid  muscles  (fig.  17). 

A  description  of  the  form  of  the  sac  in  fig.  7  will  make  the  basis 
for  the  description  of  the  peripheral  lymphatics  of  the  head,  neck  and 
thorax  worked  out  by  A.  H.  Clark  (24)  and  given  in  section  V.  The 
primitive  sac  is  the  portion  above  the  arm  bud  and  lateral  to  the 


The  Origin  and  Development  of  the  Lymphatic  System.       35 

internal  jugular  vein.  This  becomes  the  internal  jugular  trunk;  the 
cerebral  end  of  this  portion  is  an  abundant  lymphatic  plexus,  the 
anlage  of  a  single  gland  in  the  pig  (A.  H.  Clark)  which  drains  the 
pharynx.  In  the  dorsal  lymphatic  zone  the  rest  of  the  sac  is  a  com- 
plete arch  from  which  develop  the  lymphatics  of  the  shoulder,  head, 
neck,  face,  arm  and  thorax.  The  dorsal  arch  of  the  sac  becomes  by 
far  its  largest  portion,  as  will  be  readily  seen  in  fig.  8,  from  an  embryo 
3.5  cm.  long.  This  is  the  same  specimen  shown  as  fig.  3  (Sabin  130). 
It  has  since  been  cleared  by  the  Spalteholz  method,  so  that  it  shows 
the  sac  as  well  as  the  superficial  lymphatics. 

The  transition  stages  between  fig.  7  and  fig.  8  are  very  easy  to 
inject  and  can  be  readily  imagined.  The  further  development  of  the 
sac,  especially  with  reference  to  the  lymphatics  along  the  external 
jugular  vein,  is  brought  out  in  the  sections  on  the  peripheral  lymphatics 
and  glands. 

C.    THE  RENAL   SACS. 

The  study  of  the  rest  of  the  lymph  sacs  in  the  pig  may  well  begin 
with  the  interesting  and  valuable  work  of  Silvester  (140).  Silvester 
has  shown  that  in  the  monkey  all  of  the  lumbar  and  mesenteric  lym- 
phatic ducts  drain  not  by  the  thoracic  duct  into  the  jugular  veins,  but 
directly  into  the  renal  veins  or  the  surface  of  the  inferior  vena  cava 
near  the  renal  veins.  This  he  shows  in  a  number  of  figures  from 
beautiful  injections. 

A.    RETROPERITONEAL   SAC. 

It  had  previously  been  discovered  by  Lewis  (76)  in  a  study  of 
rabbit  embryos  that  a  lymphatic  sac  develops  just  ventral  to  the  median 
mesonephritic  vein.  Baetjer  (8)  proved  that  this,  the  retroperitoneal 
or  prae-aortic  sac,  arises  from  the  ventral  surface  of  the  large  vein 
which  connects  the  two  Wolffian  bodies  in  embryo  pigs  measuring  from 
17  to  23  mm.  long.  Baetjer's  fig.  3,  from  an  embryo  19  mm.  long, 
shows  the  blood-filled  lymphatic  buds  which  have  been  slightly  injected 
from  the  veins.  These  buds  rapidly  form  a  large  sac  which  lies  in  the 
root  of  the  mesentery  and  is  the  place  of  origin  for  all  of  the  mesen- 
te'ric  lymphatics  (Heuer,  43)  ;  those  that  grow  to  the  stomach  and 
intestine,  the  liver,  the  capsule  of  the  Wolffian  bodies  and  the  repro- 
ductive glands.  The  blood-filled  lymphatic  buds  can  be  seen  in  the 
fresh  embryo  pig  19  to  20  mm.  long,  if  the  intestine  is  pushed  to  the 
3 


36  Florence  R.  Sabin. 

side  and  the  veins  are  emptied  of  blood  by  injecting  them  with  salt 
solution. 

The  retroperitoneal  sac  becomes  the  largest  of  all  the  lymphatic  sacs 
in  the  pig.  As  shown  in  Heuer's  fig.  3  (43)  it  spreads  out  just  behind 
the  rectum  and  covers  the  entire  area  in  the  root  of  the  mesentery 
between  the  two  Wolffian  bodies.  It  spreads  over  the  ventral  surface 
of  the  Wolffian  bodies  as  far  as  the  edge  of  the  reproductive  glands. 
It  was  from  injecting  this  sac  that  Heuer  was  able  to  study  the  devel- 
opment of  the  lymphatics  of  the  intestine. 

This  sac  would  give  an  excellent  opportunity  to  study  the  process  of 
cavernization.  Injections  of  it  with  silver  nitrate  give  beautiful 
specimens,  showing  the  endothelial-covered  trabeculse  that  cross  its 
lumen.  Total  preparations  of  the  silvered  sac  remind  one  of  the  trabec- 
ulse in  the  wall  of  the  cavity  of  the  heart.  These  trabeculse,  which 
are  the  beginning  of  the  process  of  transferring  the  sacs  into  lymph 
glands,  show  especially  well  in  sagittal  sections  of  pig  embryos  20-25 
mm.  long. 

2.  ILIAC  SAC — CISTERNA  CHYLI. 

I  can  now  bring  some  evidence  to  show  that  the  iliac  lymphatics 
which  drain  the  legs,  tail  and  abdominal  wall,  and  the  cisterna  chyli 
which  forms  the  lower  part  of  the  thoracic  duct,  arise  together  as  buds 
from  veins  of  the  Wolffian  bodies.  A  complete  account  of  this  process 
needs  a  more  extensive  illustration  of  the  blood  vessels  of  the  region 
than  I  can  give  at  this  time.  In  the  pig  the  lymphatics  which  bud 
off  from  the  veins  of  the  Wolffian  body  and  grow  forward  dorsal  to  the 
aorta  to  form  the  cisterna  chyli  and  caudalward  along  the  edge  of  the 
Wolffian  body  to  form  the  iliac  lymphatics,  do  not  begin  until  the 
embryo  is  22  mm.  long.  I  found  that  they  arise  from  the  mesone- 
phritic  veins  because  a  direct  puncture  of  the  blood-filled  buds  entered 
the  main  veins  and  not  the  blood  capillaries.  In  a  litter  of  pigs  which 
measured  23  mm.  I  washed  out  the  blood  vessels  with  Locke's  solution, 
and  then  opened  the  specimens  and  pushed  one  of  the  Wolffian  bodies 
over  toward  the  midline.  A  plexus  of  blood-filled  lymphatics  was 
then  readily  seen  dorsal  to  the  aorta.  I  succeeded  in  puncturing  some 
of  the  larger  vessels  of  the  plexus  forming  the  cisterna  chyli  in  three 
specimens  and  saw  the  ink  run  from  the  lymphatics  into  the  mesone- 
phritic  veins.  In  all  of  these  injections  the  cisterna  chyli  is  obscured 
by  extravasations  at  the  point  of  injection,  but  they  all  show  the 


The  Origin  and  Development  of  the  Lymphatic  System.      37 

iliac  lymphatics.    For  the  present  I  shall  describe  the  renal  lymphatics 
from  sections  which  can  be  done  in  the  blood-packed  stage. 

The  description  of  the  iliac  lymphatics,  however,  needs  an  outline 
of  the  blood  vessels  of  the  region.  If  the  veins  of  the  prevertebral 
space  between  the  level  of  the  median  vein  connecting  the  Wolffian 
bodies  and  the  root  of  the  tail  be  injected  it  will  prove  that  they 


FIG.  9. — Section  through  the  hilus  of  the  Wolffian  bodies  to  show  the  place 
of  origin  of  the  iliac  lymphatic  vessels  and  the  abdominal  part  of  the 
thoracic  duct  in  an  embryo  pig  23  mm.  long.  (Specimen  23b.)  Magnified 
50  times.  A.  =  aorta;  G.  s.  s.  c.  =  gl.  suprarenalis  s.  corticalis;  L.  v.  = 
lymph  vessels  filled  with  blood;  M.  v.  =  vena  mesonephritica;  R.  s.  =  retro- 
peritoneal  lymph  sac,  a  part  of  which  is  filled  with  blood,  a  part  empty; 
W.  b.  =  edge  of  the  Wolffian  body. 


are  enormously  abundant.  In  the  embryo  measuring  20  mm.  the 
segmental  veins  draining  the  cord  and  the  body  wall  make  an 
extensive  network,  which  drains  in  part  into  the  median  vein  of  the 
Wolffian  body  and  thence  directly  to  the  vena  cava,  while  farther 
caudalward  this  same  plexus  drains  directly  into  the  large  surface 


38  Florence  R.  Sabin. 

veins  of  the  Wolffian  body.  Opposite  the  lower  pole  of  the  Wolffian 
body  this  plexus  is  directly  continued  into  the  iliac  veins.  Cross 
sections  of  injected  embryos  at  this  stage,  namely,  20  mm.,  show  that 
the  entire  area  lateral  to  the  aorta  and  between  the  two  Wolffian  bodies 
is  practically  filled  with  a  plexus  of  these  segmental  veins.  By  the 
time  the  embryo  is  22  or  23  mm.  long,  however,  there  is  a  small  non- 
vascular  area  along  the  dorsal-medial  edge  of  the  Wolffian  bodies,  and 
into  this  small  area  the  iliac  lymphatics  bud.  The  earliest  specimen 
in  which  I  have  found  these  blood-filled  buds  measures  22  mm.;  the 
specimen  is  of  interest  because  it  shows  clearly  that  the  buds  lie  against 
the  veins  of  the  Wolffian  body  itself  as  well  as  the  median  inesone- 
phritic  vein.  The  place  of  origin  of  the  cisterna  chyli  and  iliac  lym- 
phatics can  be  best  seen  in  fig.  9,  which  is  a  cross  section  of  an  embryo 
measuring  23  mm.  through  the  median  mesonephritic  vein.  Just 
lateral  to  the  aorta  are  the  masses  of  the  sympathetic  nervous  system 
through  which  runs  the  great  plexus  of  the  segmental  veins,  and  the 
cortex  of  the  adrenal  bodies.  Ventral  to  the  aorta  is  the  mesonephritic 
vein,  and  the  retroperitoneal  sac,  some  of  the  buds  of  which  are  full 
of  blood  and  some  now  partly  empty.  In  the  dorso-medial  edge  of  the 
two  Wolffian  bodies  is  the  plexus  of  blood-packed  lymphatic  buds  which 
arches  across  the  midline,  and,  as  shown  in  fig.  10,  which  is  farther 
cerebralward,  forms  a  definite  blood-filled  cisterna  chyli  in  the  midline. 
My  various  series  of  cross  sections  of  pigs  of  about  the  same  meas- 
urement are  interesting,  because  they  show  the  progression  of  the 
buds.  In  a  specimen  (23a)  the  iliac  lymphatics  extend  only  as  far  as 
the  hilum  of  the  permanent  kidney  which  lies  dorsal  to  the  lower 
pole  of  the  Wolffian  bodies.  The  iliac  buds  lie  throughout  their 
course  in  the  same  relative  position  to  the  Wolffian  bodies  and  the  aorta 
as  is  shown  in  fig.  9.  In  this  specimen  (23a)  the  iliac  lymphatic 
plexus  is  spreading  over  the  capsule  of  the  permanent  kidney,  showing 
that  it  receives  lymphatics  while  they  are  in  the  blood-filled  stage. 
In  later  stages  I  have  injected  lymphatics  from  the  iliac  sac  around 
the  pelvis  of  the  kidney.  It  is  thus  clear  why  the  lymphatics  of  the 
kidney  drain  into  the  iliac  lymph  glands,  while  those  of  the  repro- 
ductive glands  which  develop  ventral  to  the  Wolffian  body  drain 
into  the  prse-aortic  glands.  In  a  series  23b  from  the  same  litter  as  23<i 
the  iliac  plexus  is  complete,  extending  from  the  mesonephritic  vein  to 
a  dilated  sac  opposite  the  bifurcation  of  the  aorta.  It  was  this  swollen 
end  of  the  iliac  lymphatics  that  I  identified  in  embryos  25  mm.  long 


The  Origin  and  Development  of  the  Lymphatic  System.       39 

and  called  the  posterior  sac  in  1901  (Sabin  129).  The  space  for  the 
iliac  chain  in  the  pig  is  small,  much  smaller  than  in  human  embryos, 
and  the  iliac  vessels  in  the  pig  are  therefore  very  inconspicuous  in  cross 
sections,  except  in  the  blood-filled  stage.  They  can  be  found,  however, 
when  one  knows  that  they  lie  between  the  segmental  veins  and  the 
dorso-inedial  edge  of  the  Wolffian  bodies  or  kidneys.  Sagittal  sections 
are,  however,  brilliant  for  the  iliac  lymphatics  and  cisterna  chyli  in 


W;b. 


FIG.  10. — Section  through  a  plexus  of  vessels  filled  with  blood  which 
marks  the  beginning  of  the  cisterna  chyli  in  an  embryo  pig  23  mm.  long. 
(Specimen  23b.)  Magnified  40  times.  The  lymphatics  are  filled  with  blood, 
the  blood  vessels  are  empty.  A.  —  aorta;  C.  c.  —  cisterna  chyli;  G.  s.  s.  c. 
=  Gl.  suprarenalis  s.  corticalis;  G.  s.  s.  m.  =  Gl.  suprarenalis  s.  medullaris; 
M.  v.  =  vena  mesonephritica;  S.  v.  =  segmental  vein;  W.  b.  =  edge  of  the 
Wolffian  body. 


the  blood-filled  stage.  In  a  sagittal  series  of  a  specimen  23  mm.  one 
can  see  the  entire  iliac  chain  in  a  single  section,  from  the  mesone- 
phritic  vein  to  the  blood-filled  sac  just  caudal  to  the  Wolffian  body. 
From  this  caudal  swelling,  from  which  a  large  group  'of  iliac  glands 
develops  in  the  pig,  three  sets  of  vessels  grow  out:  (1)  a  plexus 
which  completely  surrounds  the  umbilical  arteries,  (2)  the  femoral 
vessels,  and  (3)  the  ileo-inguinal  vessels. 


40 


Florence  R.  Sabin. 


D.    THE  THORACIC  DUCT   ITS  THE   PIG. 

The  thoracic  duct  is  easy  to  inject  after  it  is  once  fully  formed, 
but  in  the  early  stages  exceedingly  difficult.  In  later  stages  it  can  be 
injected  indirectly  either  from  the  retroperitoneal  sac  or  from  the  iliac- 
sac.  Moreover  it  lies  in  the  edge  of  the  wall  of  the  aorta,  which  guides 
the  needle  for  a  direct  injection.  In  the  lower  thoracic  region  it 
forms  such  an  abundant  plexus  around  the  aorta  that  if  the  needle 


FIG.  11. — Section  through  the  7th  cervical  vertebra  of  an  embryo  pig 
measuring  19  mm.  Magnified  45  times.  The  blood  vessels  have  been  injected 
with  India  ink  through  the  umbilical  arteries.  This  is  the  stage  just  pre- 
ceding the  development  of  the  thoracic  duct.  B.  v.  p.  =  plexus  of  blood 
vessels  ventral  to  the  vertebra;  E.  =  esophagus;  J.  1.  s.  =  jugular  lymph 
sac;  N.  s.  =  nervus  sympatheticus;  T.  =  trachea;  V.  j.  i.  =  vena  jugularis 
interna. 

avoids  the  azygos  vein  it  is  almost  certain  to  enter  lymphatics.  Pensa's 
(105)  series  of  injections  in  different  forms,  as  well  as  fig.  7  (Heuer 
43),  show  that  it  is  a  constant  characteristic  of  the  lower  thoracic  part 
of  the  duct  to  be  a  complete  plexus  around  the  aorta,  while  the  upper 
thoracic  portion  consists  of  one  or  two  ducts. 

The  thoracic  duct  forms  from  two  places:  (1)  a  duct  which  grows 
downward  from  the  left  jugular  sac,  and  (2)  a  plexus  which  buds 
out  from  the  mesonephritic  veins  and  completely  surrounds  the  aorta. 


The  Origin  and  Development  of  the  Lymphatic  System.      41 


Before  the  thoracic  and  renal  segments  have  met,  however,  it  is  most 
difficult  to  inject  them. 

The  early  jugular  trunk  I  have  never  injected  directly  and  only 
once  indirectly  from  the  jugular  sac.  It  will  be  readily  seen  that  ink 
injected  into  the  jugular  sac  usually  takes  the  line  of  least  resistance, 
the  physiological  path  into  the  veins.  Once  in  an  embryo  23  mm. 


N.S. 
-D.-t. 


FIG.  12. — Section  through  the  point  of  origin  of  the  jugular  part  of  the 
thoracic  duct  in  an  embryo  pig  23  mm.  long.  (Specimen  23a.)  Magnified 
40  times.  The  left  jugular  lymph  sac  and  the  beginning  thoracic  duct 
emptying  into  it  were  injected  in  the  specimens;  in  the  drawing  the  veins 
are  arbitrarily  shown  as  injected  and  the  lymphatics  as  empty.  The  thick- 
ness of  the  endothelial  lining  of  the  lymphatics  has  been  exaggerated.  A.  = 
aorta;  D.  t.  =  ductus  thoracicus;  E.  =  esophagus;  N.  s.  =  nervous  sym- 
patheticus;  N.  v.  =  nervus  vagus;  J.  1.  s.  =  jugular  lymph  sac;  V.  j.  i. — 
vena  jugularis  interna. 

long  I  filled  the  sac,  and  on  pressing  the  head  forward  the  ink  shot 
into  the  thoracic  duct.  The  pressure  obviously  could  not  be  regulated. 
In  1911  I  reported  this  specimen  (23a)  at  the  meeting  of  the  Ameri- 
can Association  of  Anatomists  in  connection  with  an  unfinished  study 
of  the  thoracic  duct.  Dr.  McClure  requested  the  specimen  for  Mr. 
Kampmeier,  who  was  also  studying  the  thoracic  duct  in  the  pig  at 


42  Florence  R.  Sabin. 

the  time,  and  he  has  published,  with  my  permission,  a  valuable  recon- 
struction of  my  specimen  in  the  Anatomical  Eecord  for  1912  (Kamp- 
meier  66). 

The  thoracic  duct  needs  to  be  considered  in  relation  to  the  veins 
of  the  region.  Fig.  11  is  a  section  through  the  seventh  cervical  verte- 
bra of  an  embryo  pig  19  mm.  long,  which  is  before  the  thoracic  duct 
begins.  There  is  a  complete  vascular  injection.  It  will  be  seen  that 
there  is  a  vascular  zone  ventral  to  the  vertebra,  and  from  this  zone 
veins  surround  the  sympathetic  nerves  and  enter  the  dorso-medial 
border  of  the  internal  jugular  vein.  The  esophagus  has  a  plexus  of 
blood  vessels  in  the  submucosa,  but  dorsal  to  the  esophagus  is  a  non- 
vascular  zone  of  loose  connective  tissue.  The  jugular  stem  of  the 
thoracic  duct  grows  into  this  non-vascular  area  dorsal  to  the  esopha- 
gus, as  is  shown  in  fig.  12  from  23a.  In  this  tracing  of  the  section 
the  veins  are  arbitrarily  made  black  and  the  lymphatics  are  shown 
empty,  though  in  the  section  the  left  lymphatic  sac  and  its  ducts  were 
injected.  The  thoracic  duct  lies  in  the  margin  of  the  vascular  zone. 
As  Kampmeier's  reconstruction  of  this  specimen  shows,  there  is  a 
considerable  plexus  of  lymphatics  dorsal  to  the  esophagus  near  their 
place  of  origin  from  the  jugular  sac.  This  plexus  connects  with  the 
left  jugular  sac  in  three  places.  From  the  injected  plexus  a  short 
duct  follows  the  left  cardinal  vein  and  I  think  grows  to  the  heart 
and  lungs.  A  longer  vessel  crosses  to  the  right  side  and  is  the  jugular 
segment  of  the  thoracic  duct.  This  crossing  of  the  duct  behind  the 
aorta  I  have  frequently,  though  not  always,  noted  in  older  stages. 

The  right  lymphatic  duct  curves  ventralward  and  grows  to  the  heart 
and  lungs.  Its  course  is  shown  in  fig.  13  for  an  embryo  25  mm.  long. 
The  asymmetry  of  the  thoracic  duct  is  confined  to  the  jugular  portion 
and  corresponds  with  the  asymmetry  of  the  aorta.  It  is  interesting 
to  note  that  Sala  (137,  Taf.  14,  Fig.  16)  and  Pensa  (104,  Taf.  15, 
Fig.  2)  picture  a  symmetrical  thoracic  duct  in  the  bird. 

The  position  of  the  cisterna  chyli  has  already  been  shown  in  figures 
9  and  10.  The  cisterna  chyli  and  lower  part  of  the  thoracic  duct 
arise  in  common  with  the  iliac  sacs  from  the  mesonephritic  veins  on 
either  side,  as  shown  in  fig.  9.  These  lymphatic  buds  from  the  two 
sides  meet  dorsal  to  the  aorta  and  grow  both  cerebralward  and  caudal- 
ward  along  the  dorsal  wall  of  the  aorta.  This  makes  an  abundant 
plexus  of  blood-filled  lymphatics  along  the  course  of  the  abdominal 
aorta.  Any  sections  of  pig  embryos  of  this  stage  will  show  that  the 


The  Origin  and  Development  of  the  Lymphatic  System.      43 

veins  around  the  aorta  are  very  abundant,  so  that  the  non-vascular 
space  is  small.  Just  opposite  the  adrenal  anlage,  however,  there  is  a 
transition  zone  which  is  less  vascular.  It  marks  the  end  of  the  azygos 
veins.  Below  this  level  the  segmental  veins  drain  through  the  Wolffian 
bodies,  and  here  the  lymphatics  form  a  wider  plexus,  which  becomes 
the  cisterna  chyli,  fig.  10.  As  in  the  adult  there  is  a  gradual  tran- 


FIG.  13. — Section  through  the  upper  thoracic  region  of  an  embryo  pig 
measuring  25  mm.  in  which  the  blood  vessels  are  injected,  to  show  the 
vessels  of  the  right  lymphatic  duct  approaching  the  heart.  Magnified  40 
times.  The  injection  of  the  blood  vessels  was  made  through  the  umbilical 
artery.  A.  =  aorta;  D.  C.  =  duct  of  Cuvier;  D.  t.  =  ductus  thoracicus;  E. 
=  esophagus;  L.  =  lymph  vessels  to  the  heart  from  the  right  lymphatic 
duct;  N.  v.  =  nervus  vagus;  P.  =  pericardium;  T.  =  trachea;  V.  a.  = 
vena  azygos. 

sition  between  the  cisterna  chyli  and  the  rest  of  the  aortic  plexus, 
both  the  part  which  extends  caudalward  and  the  part  which  extends 
cerebralward.  Figure  10  shows  the  cisterna  chyli  while  it  is  still  a 
plexus  and  not  a  sac.  The  series  shows  that  in  many  places  this  dorsal 
plexus  of  lymphatics  is  sending  sprouts  around  to  the  ventral  prae- 
aortic  sac.  All  of  the  abdominal  sacs,  the  retroperitoneal,  ventral  to 
the  aorta,  the  two  lateral  iliac  sacs  and  the  dorsal  plexus  connect  with 


44  Florence  R.  Sabin. 

each  other.  In  following  the  series  cerebralward  from  the  level  of  fig.  9 
it  is  clear  that  the  blood-filled  lymphatics  not  only  arch  across  the 
midline  dorsal  to  the  aorta,  but  they  follow  the  border  of  the  Wolffian 
bodies  lateral  to  the  aorta.  Above  the  adrenal  bodies  the  Wolffian 
bodies  approach  very  close  to  the  aorta,  and  some  of  the  lymphatic 
buds  curve  around  the  ventral  surface  of  the  aorta.  Thus  in  a  speci- 
men 23  mm.  long  there  is  a  plexus  of  blood-filled  lymphatics  surround- 
ing the  aorta  and  extending  into  the  lower  thorax  as  far  as  the  cerebral 
pole  of  the  Wolffian  bodies.  At  the  cerebral  end  of  the  Wolffian  bodies 
there  are  large  veins  which  curve  ventral  to  the  aorta  and  connect 
the  veins  of  the  capsule  of  the  Wolffian  bodies.  Some  of  the  lymphatics 
are  near  these  median  veins. 

The  finding  of  the  renal  lymphatics  in  the  blood-packed  stage  makes 
it  quite  certain  that  the  thoracic  duct  arises  in  two  places:  First, 
the  jugular  stem,  which  can  be  injected  from  the  left  jugular  sac  and 
develops  long  after  the  jugular  sacs  are  entirely  empty  of  blood;  and, 
second!}-,  the  renal  plexus,  which  surrounds  the  aorta  and  forms  a  defi- 
nite cisterna  chyli. 

These  two  observations,  namely,  (1)  the  presence  of  blood  in  one 
part  and  its  absence  in  the  other,  and  (2)  the  fact  that  injections 
prove  a  connection  of  the  jugular  stem  with  the  jugular  sac,  and  of 
the  abdominal  plexus  with  the  mesonephritic  veins,  make  it  quite  cer- 
tain that  the  two  portions  of  the  thoracic  duct  are  distinct  from  each 
other  in  embryo  pigs  measuring  23  mm.,  and  hence  that  the  thoracic 
duct  begins  in  two  places.  Practically  the  entire  question  at  issue 
between  those  of  us  who  think  that  the  lymphatics  grow  by  their  own 
endothelial  wall  and  those  who  do  not  centers  around  the  question 
of  how  the  thoracic  duct  develops,  or,  to  put  it  more  specifically,  how 
these  two  lymphatic  anlagen  become  connected.  The  discussion  centers 
around  my  specimen  23a,  which  Kampmeier  studied  and  which  he 
uses  as  his  most  conclusive  evidence  of  the  theory  of  the  growth  of 
lymphatics  by  the  addition  of  tissue  spaces.  On  the  other  hand,  it 
is  my  theory  that  the  thoracic  duct  grows  from  these  two  anlagen  from 
the  veins  by  the  same  method  by  which  other  lymphatic  capillaries 
can  be  seen  to  grow  in  a  living  specimen,  namely,  by  the  sprouting  of 
their  endothelial  wall.  It  is  true  that  I  have  not  yet  sufficiently 
mastered  the  difficulties  of  injecting  the  renal  lymphatics  to  demon- 
strate the  progression  of  the  thoracic  duct  and  indeed  it  may  not 
prove  possible  to  get  as  conclusive  proof  of  the  origin  of  the  deep  lym- 


The  Origin  and  Development  of  the  Lymphatic  System.      45 

phatic  sacs  as  of  the  more  superficial  ones;  yet  the  inference  that 
they  arise  in  the  same  manner  is  in  harmony  with  our  modern  knowl- 
edge of  morphology.  I  wish  to  postpone  a  discussion  of  Kampmeier's 
evidence  until  I  have  described  the  thoracic  duct  in  a  human  embryo, 
and  discussed  the  development  of  the  theories  opposed  to  my  own — of 
which  Kampmeier's  work  is  a  part  (Sec.  VIII). 

2.  PRIMARY  LYMPHATIC  SYSTEM  IN  HUMAN  EMBRYOS. 

On  this  description  of  the  lymphatic  system  in  one  mammalian  form 

1  shall  base  an  account  of  what  is  known  of  the  primary  system  in  other 
forms.     All  of  the  primary  lymphatic  sacs  have  been  found  in  the 
human  embryo.     The  jugular  sac  is  the  earliest  to  appear.     It  has 
been  found  as  a  few  blood-packed  buds  in  two  embryos  measuring  8 
and  9  mm.    (Mall  collection,  No.  397  and  No.  163,  Sabin  134.)    They 
are  on  the  lateral  surface  of  the  anterior  cardinal  vein  near  the  duct  of 
Cuvier  and  were  described  as  veins  (Sabin  133,  134),  since  at  that 
time  the  blood-filled  buds  were  thought  to  be  blood  capillaries.    The 
blood-packed  buds  extend  along  the  lateral  surface  of  the  anterior  car- 
dinal vein  and  very  early  develop  sprouts  that  project  dorsalward. 
These  are  plain  in  an  embryo  measuring  11  mm.     (Mall  collection,  No. 
353,  figs.  7  and  8,  Sabin  133,  and  figs.  490  and  491,  Sabin  134.)    The 
figures  of  this  specimen,  together  with  the  reconstruction  of  Lewis 
(Harvard  collection  No.  1000,  measuring  16  mm.;  No.  189,  measuring 
11.5  mm.;  and  No.  1322,  measuring  16  mm.,  figs.  1-3,  Lewis  78), 
show  that  the  jugular  lymphatics  in  the  human  embryo  not  only 
bud  from  the  anterior  cardinal  veins,  but  from  the  posterior  cardinal 
vein,  and  the  common  stem  or  plexus  which  forms  the  origin  of  the 
primitive  ulnar  and  thoraco-epigastric  veins.     A  large  extension  of 
the  jugular  sac  along  the  primitive  ulnar  vein  is  characteristic  of 
human  embryos  (fig.'  12,  Sabin  133,  or  fig.  493,  Sabin  134,  and  figs. 

2  and  3  after  Lewis  78). 

In  a  human  embryo  10.5  mm.  long  I  have  found  a  small  sac  only 
partly  filled  with  blood  (figs.  3-6,  Sabin  133,  and  figs.  488,  489,  Sabin 
134).  This  specimen  has  also  a  beginning  thoracic  duct.  These  two 
facts  show  that  there  is  considerable  variation  in  the  rate  of  growth 
of  the  lymphatic  system  of  different  specimens. 

The  study  of  human  embryos  has  also  brought  out  the  fact  that 
the  openings  of  lymphatics  into  the  veins  can  only  be  made  out  when 


46  Florence  R.  Sabin. 

the  section  happens  to  be  cut  in  the  right  plane  for  each  valve.  For 
the  jugular  sac  frontal  sections  are  the  best  (figs.  493,  494,  Sabin  134) . 

A  new  embryo  in  the  Mall  collection,  Xo.  460,  measuring  21  mm., 
is  of  great  interest  in  connection  with  the  renal  lymphatics.  A  care- 
ful reconstruction  of  this  specimen  with  abundant  illustrations  would 
be  of  value,  but  for  the  present  I  can  give  only  a  description.  I  in- 
jected the  embryo  with  India  ink  into  the  umbilical  artery  while  the 
heart  was  still  beating.  The  vascular  injection  is  almost  perfect.  The 
embryo  was  then  put  directly  into  bichloride-acetic  and  the  fixation 
is  excellent. 

In  marked  contrast  to  the  embryo  pig  of  about  the  same  length,  the 
Wolffian  bodies  are  disappearing,  being  pushed  caudalward  by  the 
growing  permanent  kidneys.  The  cerebral  pole  of  the  Wolffian  bodies 
lies  far  to  the  side  opposite  the  median  mesonephritic  vein  which,  as 
Baetjer  (8,  fig.  7)  shows,  connects  the  Wolffian  bodies  opposite  their 
hilum  in  embryo  pigs  measuring  20  mm.  This  median  vein  in  the 
human  embryo  (No.  460)  passes  over  the  ventral  surface  of  the  adrenal 
bodies  and  still  receives  the  veins  of  the  cerebral  pole  of  the  W^olffian 
bodies.  It  also  receives  a  plexus  of  veins  from  the  permanent  kidneys. 
It  is  markedly  asymmetrical  on  account  of  the  development  of  the 
vena  cava  on  the  right  side. 

Blood-filled  lymphatic  buds  completely  surround  this  mesonephritic 
vein  in  the  midline,  making  the  anlage  of  the  retroperitoneal  or 
prae-aortic  sac ;  some  of  those  that  come  from  the  dorsal  surface  of  the 
vein  have  pushed  between  the  masses  of  the  sympathetic  ganglia  and 
reached  the  dorsal  surface  of  the  aorta.  In  some  sections,  the  retro- 
peritoneal  buds  are  partly  emptied  of  their  blood.  The  area  at  the 
root  of  the  mesentery  opposite  the  median  mesonephritic  vein  is  small 
in  the  human  embryo  as  compared  with  the  corresponding  area  in  the 
pig  and  the  retroperitoneal  sac  is  correspondingly  small. 

The  iliac  sacs  and  the  beginning  thoracic  duct  are  also  present. 
Along  the  dorso-medial  wall  of  the  kidney  in  the  angle  between  the 
segmental  veins  and  the  plexus  of  renal  veins  are  two  long  iliac  sacs. 
They  are  evidently  farther  advanced  than  the  retroperitoneal  sac,  for 
they  are  nearly  empty.  The  one  on  the  left  side  measures  1.8  mm. 
and  extends  to  the  bifurcation  of  the  aorta.  On  the  right  side  the 
empty  sac  is  considerably  shorter,  but  its  lower  part  is  replaced  by 
blood- filled  buds  along  the  inferior  vena  cava. 


The  Origin  and  Development  of  the  Lymphatic  System.       47 

In  tracing  the  lymphatics  cerebralward,  from  the  level  of  the  hilus 
of  the  kidney,  there  is  a  small  lymphatic  capillary  plexus  in  the  place 
of  the  cisterna  chyli  and  the  two  lateral  sacs  become  a  plexus  of  small 
ducts  which  can  be  traced  the  entire  length  of  the  permanent  kidneys 
and  then  along  the  dorso-lateral  surface  of  the  aorta,  just  ventral  to  the 
azygos  veins.  This  small  plexus  of  ducts  is  nearly  empty  on  the  left 
side,  but  full  of  blood  on  the  right  side.  The  two  plexuses  can  be 
followed  almost  to  the  level  of  the  bifurcation  of  the  trachea.  Some 
blood  in  the  plexus  on  the  left  side  just  before  the  vessels  end  makes 
it  possible  that  the  renal  part  of  the  thoracic  duct  does  not  yet  connect 
with  the  jugular  part  of  the  duct. 

This  particular  specimen  is  interesting  in  regard  to  certain  zones 
of  dilated  tissue  spaces  which  are  conspicuous  along  the  aorta.  There 
are  some  of  these  large  spaces  near  the  bifurcation  of  the  aorta,  in 
the  root  of  the  mesentery  near  the  retroperitoneal  sac,  and  along  the 
ventral  surface  of  the  aorta,  especially  opposite  the  mesentery  of  the 
stomach  and  opposite  the  bifurcation  of  the  trachea.  I  have  studied 
them  with  care  to  see  if  they  have  any  definite  relation  to  the  develop- 
ing lymphatics  and  am  sure  that  they  have  not.  Sometimes  they  are 
near  the  lymphatics  and  sometimes  not.  Those  in  the  thorax  are  not 
adjacent  to  the  lymphatic  plexus.  They  can  be  distinguished  from 
the  endothelial-lined  lymphatics,  but  if  one  were  convinced  that  lym- 
phatics came  from  tissue  spaces  one  might  imagine  transition  pictures 
between  the  tissue  spaces  and  the  ducts.  This  is  particularly  true  when 
the  endothelium  of  a  lymphatic  vessel  sags  away  from  the  surrounding 
tissue,  a  picture  familiar  to  histologists  in  sections  showing  lymphatics 
in  adult  tissues.  The  further  discussion  of  the  thoracic  duct  is  in 
part  VIII. 

•  All  of  the  primary  lymphatic  system  is  present  in  a  human  embryo 
measuring  30  mm.  (Mall  collection  No.  86,  fig.  12,  Sabin  133,  copied  as 
fig.  493,  Strieker  144) .  This  stage  represents  the  maximum  size  of  the 
jugular  sac.  The  dorsal  arch  is  large  and  its  line  of  separation  from 
the  jugular  part  is  indicated  by  the  perforation  for  the  cervical  nerves 
and  blood  vessels.  The  very  small  mesenteric  sac  and  larger  iliac  sacs 
are  distinct  as  well  as  the  complete  thoracic  duct. 

3.  PRIMARY   LYMPHATIC  SYSTEM  isr  EABBIT  EMBRYOS. 
In  1906  F.  T.  Lewis  (76)  published  a  valuable  paper  on  the  lym- 
phatic system  in  rabbit  embryos.    Besides  excellent  reconstruction  of 


48  Florence  R.  Sabin. 

the  primary  lymphatic  system  in  rabbits,  the  cat  and  the  pig,  this  paper 
has  three  important  points:  First,  the  discovery  that  the  early  lym- 
phatics are  filled  with  blood,  which  has  proved  to  be  of  such  value, 
now  that  its  meaning  is  understood.  Second,  the  discovery  of  the 
retroperitoneal  sac  which  he  described  as  lying  adjacent  to  the  mesen- 
teric  veins.  He  also  showed  the  iliac  sacs  and  cisterna  chyli;  all  of 
which  we  now  know  come  from  the  renal  veins  (Sabin  136).  Third, 
he  noted  that  lymphatic  ducts  when  they  are  reconstructed  from  sec- 
tions appear  as  a  row  of  beads.  Since  these  rows  of  beads  occurred 
along  the  veins  in  his  reconstructions  and  since  he  could  not  find  the 
connections  of  the  early  sacs  with  the  veins  he  was  led  to  suggest  that 
lymphatics  might  arise  from  multiple  anlagen  split  off  from  the  veins 
along  their  course.  The  apparently  isolated  endothelial-lined  vessels 
found  in  serial  sections  we  call  Meyer-Lewis  anlagen.  They  are  dis- 
cussed in  section  VIII. 

The  jugular  sac  in  rabbits  has  an  early  extension  along  the  primitive 
ulnar  vein  like  that  of  the  human  embryo.  The  dorsal  portion  of  the 
sac,  for  the  posterior  triangle  of  the  neck,  arises  from  a  large  stalk 
from  the  primary  jugular  portion  between  the  level  of  the  4th  and  5th 
cervical  nerves. 

4.  PRIMARY  LYMPHATIC  SYSTEM  IN  THE  CAT. 

The  primary  lymphatic  system  in  the  cat  will  be  described  from  the 
extensive  work  of  Huntington  and  McClure  (51-58  and  91-94),  one 
reconstruction  of  Lewis  (76)  and  a  few  injections  of  my  own.  The 
work  of  Huntington  and  McClure  has  been  mainly  on  the  development 
of  the  jugular  lymph  sacs  and  the  thoracic  duct  in  the  cat  and  their 
observations  are  for  the  most  part  brought  together  in  two  extensive 
papers  (54  and  58).  Their  work  is  difficult  to  analyze  and  cannot 
be  done  entirely  accurately  without  seeing  their  sections.  They  have 
relied  on  the  appearances  of  sections  and  the  appearances  of  wax  models 
from  them  to  differentiate  lymphatics,  and  since  these  are  not  ade- 
quate criteria  it  is  impossible  to  be  sure  that  all  the  structures  shown 
as  lymphatics  are  lymphatics;  in  fact,  one  can  be  certain  that  some  of 
them  are  not.  Their  theories  in  regard  to  the  origin  and  development 
of  the  lymphatic  system  are  discussed  in  sec.  VIII.  In  the  article 
(54)  on  the  jugular  lymph  sac  of  the  cat  figs.  8  to  21  may  be  disre- 
garded as  evidence,  since  they  are  diagrams.  Figure  22,  which  they 


The  Origin  and  Development  of  the  Lymphatic  System.      49 

group  under  "  Early  Venous  Stages,"  is,  I  think,  a  reconstruction  of 
the  cardinal  veins  before  the  lymphatics  begin  (and  is  therefore  to 
be  compared  with  Evans'  (34)  fig.  3,  and  with  my  fig.  4  in  this  paper) . 
In  their  fig.  22  the  fourth  segmental  vessel  is  shown  and  described 
(page  226)  as  "  formed  by  the  confluence  of  two  extended  para-neural 
channels."  This  is  intended  to  indicate  the  beginning  of  the  lym- 
phatics, but  since  their  fig.  43  from  an  embryo  10  mm.  long  shows 
undoubted  lymphatics  in  the  blood-packed  stage,  I  think  it  is  prac- 
tically certain  that  the  specimen  measuring  5  mm.  does  not  show  lym- 
phatics. It  is  not  necessary  to  prove  that  veins  do  not  open  into  the 
tissue  spaces  around  nerves. 

The  description  of  the  stages  between  5  and  10  mm.  is  bound  up 
with  the  discussion  of  "  fenestration."  In  reconstructing  the  veins 
Huntington  and  McClure  have  noted  the  pattern  of  the  developing 
veins,  which  is  especially  well  shown  in  fig.  4,  in  connection  with  the 
pericardial  veins.  In  this  specimen  the  pericardial  veins  drain  into  a 
loop  which  connects  the  anterior  cardinal  vein  with  the  duct  of  Cuvier. 
The  formation  of  this  loop  is  a  part  of  the  process  by  which  the  peri- 
cardial veins  shift  from  the  duct  of  Cuvier  to  the  anterior  cardinal 
vein.  Numerous  small  loops  are  seen  along  the  anterior  cardinal 
veins  in  figs.  4  and  5.  They  are  characteristic  of  the  veins.  They 
suggested  to  Huntington  and  McClure  a  vague  conception  of  a  rela- 
tion to  the  lymphatics  which  they  expressed  as  follows  (p.  204  in  54)  : 
"  In  a  subsequent  stage  confluence  of  these  '  f  enestrae '  results  in  the 
more  or  less  complete  separation  of  the  primary  vein  into  two  parallel 
secondary  channels,  which  appears  to  be  a  uniform  principle  in  the 
development  of  parallel  venous  trunks.  Or,  as  in  the  case  of  the  jugu- 
lar lymph  sac,  further  extension  of  the  same  process  may  result  in 
separating  from  the  main  venous  channels  elements  which  unite  to 
form  a  closed  sac  entirely  distinct  from  the  vein  from  which  it  arose. 
The  term  '  fenestration  '  means,  therefore,  in  the  sense  in  which  it  is  em- 
ployed in  this  paper,  one  of  the  last  stages  in  the  definite  crystalli- 
zation of  the  venous  system  out  of  an  indefinite  antecedent  plexiform 
condition,  and  the  determination  of  an  important  element  in  lymphatic 
organization,  closely  associated  with  the  embryonic  venous  system." 
The  lymphatics  which  they  thought  formed  by  this  process  they  termed 
"  veno-lymphatics."  Both  terms  "  fenestration  "  and  "  veno-lymphat- 
ics  "  should  drop  out ;  there  are  no  veno-lymphatics,  for  the  lymphatics 
bud  directly  from  the  veins  as  lymphatics,  and  fenestration  is  a  term 


50  Florence  R.  Sabin. 

that  describes  models  better  than  the  actual  process  of  growth  by  sprout- 
ing by  which  the  veins  are  formed. 

Some  of  the  figures  of  the  models  from  embryos  measuring  7  mm., 
for  example  fig.  33,  represent  lymphatics  in  the  early  plexus  stage; 
I  do  not,  however,  find  any  mention  of  the  presence  of  blood.  Figure 
46  shows  the  beginning  of  the  enlarging  of  the  plexus  into  sacs.  From 
their  later  stages  figs.  46  to  65,  but  much  better  from  Lewis,  fig.  6 
(76),  can  be  made  out  the  especial  characteristics  of  the  jugular 
sacs  in  the  cat.  They  are  (1)  that  the  jugular  sac  in  the  cat  arises 
from  the  posterior  cardinal  vein  as  well  as  from  the  anterior  cardinal 
vein.  This  point  is  well  shown  in  Huntington  and  McClure  figures. 
(2)  The  cerebral  end  of  the  ventral  or  jugular  portion  of  the  sac  is 
very  large.  This  is  the  part  that  drains  the  larynx.  (3)  The  internal 
jugular  trunk  is  small.  (4)  The  dorsal  apex  of  the  sac  which  lies 
in  the  posterior  triangle  of  the  neck  is  large.  (See  Lewis'  fig.  6.)  It 
is  connected  with  the  cerebral  end  by  a  slender  vessel.  In  injections 
of  the  sac  in  embryos  between  30  and  60  mm.  long  it  looks  very  much 
like  the  sac  in  a  bovine  embryo  as  shown  by  Polinski  (107)  in  his  fig. 
7.  It  lies  nearer  the  arm  than  the  corresponding  portion  of  the  sac 
in  the  pig.  (5)  There  is  a  large  extension  though  possibly  a  dis- 
tinct sac  (Lewis,  fig.  6)  along  the  primitive  ulnar  and  thoraco-epi- 
gastric  veins.  This  sac  is  very  conspicuous  in  older  stages,  extending 
into  the  axilla  and  receiving  large  trunks  from  the  thoracic  wall. 
This  makes  the  jugular  lymphatic  sac  in  the  cat  more  like  the  human 
than  the  pig  (sec.  V). 

The  segmental  divisions  of  the  sac  as  indicated  by  colors  in  the 
Huntington  and  McClure  figures  are  arbitrary  and  not  as  valuable 
as  the  divisions  I  have  just  given,  which  correspond  to  the  function 
of  the  different  glands  which  develop  from  the  sac. 

Lewis'  figure  also  shows  a  very  interesting  view  of  the  retroperito- 
neal  and  iliac  sacs  as  a  continuous  plexus,  which  again  emphasizes 
the  fact  that  the  renal  sacs  anastomose  with  each  other. 

5.  PRIMARY  LYMPHATIC  SYSTEM  IN  BIRDS,  POSTERIOR  LYMPH 
HEART  AND  JUGULAR  LYMPH  SAC. 

The  history  of  our  knowledge  of  the  lymphatic  system  in  birds  is 
most  interesting.  The  early  discoveries  of  the  posterior  lymph  hearts 
by  Panizza,  A.  F.  J.  Mayer,  and  Stannius,  as  well  as  the  work  of 


The  Origin  and  Development  of  the  Lymphatic  System.       51 

Budge  on  the  relation  of  the  heart  to  the  allantoic  vessels,  have  already 
been  given.  From  that  time  on  there  was  no  advance  until  the  work 
of  Sala  (137),  who  studied  the  development  of  the  posterior  lymph 
hearts  and  thoracic  duct.  He  described  the  posterior  lymph  hearts 
as  arising  in  relation  to  the  lateral  branches  of  the  first  five  coccygeal 
veins,  during  the  seventh  day  of  incubation.  We  now  know  that 
the  posterior  lymph  hearts  bud  from  the  veins  during  the  5th  day 
(E.  E.  and  E.  L.  Clark,  29),  so  that  Sala  was  describing  the  trans- 
formation of  the  plexus  into  the  sac.  He  noted  the  connections  with 
the  veins  but  described  the  sac  as  arising  from  excavations  in  the 
mesenchyme.  His  .work,  published  a  short  time  before  my  paper  on 
the  origin  of  the  lymphatic  system  from  the  veins  (Sabin  129),  was  an 
advance  over  the  preceding  work,  for  it  placed  the  first  lymphatics 
near  the  veins  instead  of  in  the  periphery.  Sala  figured  the  thoracic 
duct  in  the  bird  as  a  symmetrical  structure  (Taf.  14,  Fig.  16),  and 
this  is  also  shown  in  Pensa  (104,  Taf.  15,  Fig.  2).  Sala  described 
the  early  stages  of  the  thoracic  duct  as  being  solid  cords  of  cells.  This 
observation,  which  has  been  confirmed  by  'Mierzewski  (96),  makes  it 
necessary  to  restudy  the  thoracic  duct  in  the  chick,  which  is  now  pos- 
sible through  the  great  improvement  in  our  methods  of  injection. 

The  most  recent  work  on  the  lymphatic  system  in  birds  has  been 
done  by  Mierzewski  (96),  Jolly  (59),  Miller  (97)  and  E.  E.  and 
E.  L.  Clark  (27-29). 

I  have  already  brought  out  the  fundamental  importance  of  the 
darks'  work,  showing  that  the  lymphatics  bud  off  from  the  veins  in 
a  non-vascular  zone,  and  show  a  continuous  growth  to  the  periphery. 
Mierzewski  had  shown  that  the  early  lymphatics  grow  along  the  lateral 
line  of  the  embryo,  as  shown  in  his  fig.  3.  It  was  these  lymphatics 
which  grow  to  connect  the  posterior  lymph  heart  with  the  jugular 
lymph  sac  that  E.  L.  Clark  observed  in  their  blood-filled  stage  and 
which  led  her  to  watch  the  process  of  budding  in  the  living  embryo. 

The  process  may  best  be  described  in  a  quotation  from  p.  254  (29)  : 

The  first  evidence  of  lymphatics  in  the  tail  region  of  living  chicks  is  the 
appearance  of  a  number  of  separate  knobs,  filled  with  stagnant  blood,  a 
little  darker  in  color  than  the  circulating  blood,  just  lateral  to  several  of  the 
most  anterior  of  the  dorsal  intersegmental  coccygeal  veins.  The  connections 
with  the  veins  cannot  be  seen,  since  the  knobs  lie  between  them  and  the  ob- 
server, but  ink  injected  into  the  knobs  can  be  seen  to  pass  directly  into  the 
main  intersegmental  veins.  Between  the  separate  knobs  no  anastomoses 
can  be  seen,  nor  can  any  be  discovered  by  injection. 


52 


Florence  R.  Sabin. 


Soon  after  these  knobs  appear  (in  about  fifty-five  minutes),  similar  ones 
develop  about  them  which  have  fine  connections  with  them,  thus  forming 
a  small  cluster.  The  new  ones  are  located  partly  on  either  side  of  the  first 
and  partly  superficial  to  them.  Their  injection  now  shows  discreet  tiny 
clusters,  somewhat  like  bunches  of  grapes,  connected,  as  were  the  earliest 
knobs,  with  the  same  intersegmental  veins.  These  clusters  are  still  separate 
from  one  another. 


FIG.  14. — After  Miller.  Diagram  from  an  reconstruction  of  the  veins  and 
nerves  of  the  cervical  and  upper  thoracic  regions  of  a  chick  embryo  13.5 
mm.  long,  after  5  days  and  10  hours  of  incubation;  right  side.  1.  Prae- 
cardinal  vein.  2.  Post-cardinal  vein.  3.  Duct  of  Cuvier.  4.  Intersegmental 
(dorsal)  veins.  5.  Lateral  group  of  vascular  islands  and  veno-lymphatic 
vessels.  9.  Spinal  (cervical)  nerves.  9a.  Brachial  plexus.  After  Miller 
Amer.  Jour  of  Anat.,  1912,  Vol.  12,  tig.  12. 


The  Origin  and  Development  of  the  Lymphatic  System.       53 

-C.L.D. 


? — T.D 


FIG.  15. — After  E.  L.  Clark.  Lateral  view  of  the  jugular  lymphatic  plexus 
of  the  right  side  of  a  chick  embryo  14  mm.  long  after  an  incubation  of  5 
days  and  20  hours.  The  measurement  was  the  greatest  length  and  was 
taken  after  fixation.  With  a  very  fine  canula  (the  lumen  of  the  point  being 
about  15  M  in  diameter)  diluted  India  ink  was  injected  under  a  binocular 
microscope  into  one  of  the  superficial  lymphatic  capillaries  in  the  skin 
between  the  fore  and  hindlegs.  From  here  the  ink  filled  the  deep  jugular 
lymphatic  plexus,  and  from  there  a  few  granules  entered  the  vein  through 
five  connections  (C).  The  drawing  was  made  with  the  help  of  a  camera 
lucida  with  a  Zeiss  binocular  microscope  (oculars  No.  4,  objective  A8). 
Magnified  67  times.  A.  =  vessel  which  connected  the  superficial  lymphatic 
capillaries  with  the  jugular  lymph  plexus  and  which  was  injected;  A.  C.  = 
vena  cardinalis  anterior;  P.  C.  =  vena  cardinalis  posterior;  D.  C.  =  duct 
of  Cuvier;  C.  =  communications  between  the  jugular  lymphatic  plexus  and 
the  veins;  T.  D.  =  deep  lymphatic  vessel;  T.  E.  V.  =  vena  thoraco-epi- 
gastrica;  C.  L.  D.  —  cervical  lymph  vessel.  After  E.  L.  Clark's  fig.  1,  Anat. 
Record,  1912,  Vol.  6,  p.  263. 


54  Florence  R.  Sabin. 

There  is  a  rapid  extension  of  these  blood-filled  structures,  and  soon,  in 
about  an  hour  and  a  half  after  their  first  appearance,  connections  between 
neighboring  clusters  may  be  seen.  Injection  at  this  stage  with  India  ink 
shows  an  anastomizing  plexus,  connected,  as  before,  with  the  intersegmental 
veins.  Injection  with  silver  nitrate  shows  distinct  endothelial  markings 
in  the  walls  of  the  plexus. 

During  this  plexus  formation  there  is  a  steady  extension  toward  the  sur- 
face, and  by  the  time  anastomoses  have  formed  between  neighboring  clusters 
sprouts  have  grown  to  the  surface  and  started  to  extend  in  the  region 
superficial  to  the  plexus  and  also  ventralwards.  It  now  becomes  possible 
to  study  with  more  minuteness  the  changes  which  are  going  on,  since  these 
sprouts  are  quite  superficial  and  are  developing  in  a  plane  parallel  with 
the  surface.  Observation  and  successive  records  of  these  sprouts  in  the 
living  chick  reveal  a  rapid  extension  ventrally  and  also  anteriorly,  ac- 
companied by  a  plexus  formation.  Two  or  three  sprouts  are  seen  to  lead  and 
soon  numerous  connections  develop  between  them.  Various  portions  of  the 
irregular  plexus  thus  formed  enlarge  and  become  more  densely  packed  with 
blood  which  continues  to  back  up  from  the  vein.  Then  new  sprouts  grow 
out  in  advance  and  the  same  process  of  extension  accompanied  by  plexus 
formation  is  repeated.  If  a  single  sprout  is  selected  and  frequent  careful 
drawings  are  made  the  changes  are  seen  to  be  rapid  and  striking.  The 
sprout  becomes  wider  and  longer.  Branches  appear,  and  they  in  turn  in- 
crease in  width  and  length.  From  a  branch  a  connection  forms  with  the 
original  sprout,  thus  forming  a  loop.  New  branches  and  connections  are 
formed,  making  a  plexus.  Branches  from  neighboring  loops  or  plexuses 
meet  one  another  and  anastomose.  The  several  parts  of  the  plexus  are 
quite  irregular  in  size.  Most  of  the  lymphatic  vessels  are  several  times  as 
wide  as  a  blood  capillary,  while  some  of  the  connections  are  as  small  as, 
or  even  smaller,  than  a  blood  capillary.  Throughout  the  blood  in  these  new- 
forming  lymphatics  is  markedly  darker  in  color  than  the  circulating  blood. 

This  work  thus  corrects  the  idea  that  lymphatics  are  transformed 
blood  capillaries.  They  bud  out  from  the  veins  as  lymphatics  and 
gradually  invade  the  body  as  a  new  type  of  vessels. 

Miller's  work  (97)  on  the  anterior  lymph  sac  is  chiefly  valuable 
in  giving  an  excellent  chance  to  contrast  the  method  of  reconstruction 
with  the  method  of  injection  in  an  area  where  an  injection  can  be 
obtained.  Figure  14  is  copied  from  Miller's  fig.  6,  and  fig.  15  is  from 
an  injection  by  E.  L.  Clark's  (27,  fig.  2)  of  the  plexus  in  the  same 
stage.  The  points  of  the  apparently  isolated  islands  of  lymphatics 
and  the  separation  of  the  sac  from  the  veins  show  by  Miller  are  errors 
which  are  cleared  up  by  E.  L.  Clark's  work.  The  jugular  lymphatic 
plexus  is  connected  with  the  vein  in  a  number  of  places  and  is  a  con- 
tinuous endothelial-lined  plexus  of  lymphatics. 


The  Origin  and  Development  of  the  Lymphatic  System.       55 

The  jugular  lymph  plexus  develops  into  a  small  sac  ('Miller's  fig.  6) 
which  becomes  a  single  jugular  gland.  This  small,  fusiform  gland 
has  been  studied  by  a  number  of  observers  and  has  been  worked  out 
in  all  its  relations  by  Jolly  (59)  in  his  interesting  paper  on  the  lymph 
glands  in  buds. 

Miller  (97,  page  489)  quotes  Mierzewski  as  having  seen  rhythmical 
contractions  of  the  jugular  lymph  sacs.  A  reference  to  Mierzewski's 
paper,  however,  will  show  that  he  consistently  speaks  of  posterior 
lymph  heart  and  jugular  lymph  plexus  and  his  reference  to  pulsation 
(page  479)  is  a  confirmation  of  Sala's  observations  in  the  posterior 
lymph  hearts.  Miller  notes  the  absence  of  muscle  and  the  point  should 
be  emphasized,  because  the  jugular  lymph  sac  in  birds  becomes  not 
a  lymph  heart,  but  a  lymph  gland. 

The  iliac  lymphatics  in  birds  are  very  well  brought  out  by  Jolly 
(59)  in  his  article  on  lymph  glands.  He  does  not  consider  their 
origin,  but  his  fig.  15  shows  their  distribution  extremely  well,  and  his 
fig.  11,  pi.  XI,  gives  the  exact  histological  picture  of  a  lymph  sac  with 
connective  tissue  bridges  fon  a  bird  on  the  18th  day  of  incubation. 
This  is  the  method  by  which  the  primary  sacs  become  lymph  glands, 
as  is  shown  in  section  VI. 


6.  PRIMARY  LYMPHATIC  SYSTEM  IN  AMPHIBIA,  ANTERIOR  AND 
POSTERIOR  LYMPH  HEARTS. 

It  is  not  possible  at  this  time  to  give  a  complete  account  of  the 
development  of  the  lymphatic  system  of  the  amphibia  and  reptilia, 
but  enough  evidence  is  at  hand  to  show  that  the  lymphatics  are  derived 
from  the  veins.  The  remarkable  subcutaneous  lymph  sacs  in  the  frog 
have  always  attracted  interest  and  stimulated  a  curiosity  to  unravel 
their  development  as  was  brought  out  by  Langer  (75)  in  1868.  Some 
recent  experiments  of  Abel  (1)  which  combine  anatomical  and  pharma- 
cological methods  add  to  this  interest. 

Meltzer  (95)  had  published  experiments  to  show  that  a  smaller  dose 
of  a  drug  produced  convulsions  when  injected  into  a  cardiectomized 
frog  than  into  a  normal  one.  He  concluded  that  the  drug  traveled 
through  the  tissue  spaces. 

Abel  found  that  a  dye,  acid  fuchsin,  when  injected  into  the  lymph 
sacs  of  a  normal  frog  was  distributed  widely  over  the  body  by  the 
blood  vessels.  In  a  cardiectomized  frog  a  small  dose  injected  into  the 


56  Florence  R.  Sabin. 

muscle  (where  there  are  no  lymphatics,  only  tissue  spaces)  traveled 
exceedingly  slowly;  on  the  other  hand,  a  small  amount  of  the  fluid- 
injected  into  a  lymph  sac  traveled  rapidly,  but  in  a  zone  limited  ex- 
clusively to  the  lymph  sacs,  according  to  their  anatomical  conections. 
When  the  dye  reached  the  lymph  heart  in  the  cardiectomized  frog 
it  was  pumped,  not  through  the  heart  to  the  systemic  vessels,  hut 
through  the  vertebral  veins  to  the  nervous  system.  Thus  is  explained 
the  marked  effect  of  the  small  dose.  If  the  lymph  hearts  were  killed 
the  convulsions  did  not  take  place. 

It  has  been  shown  that  the  anterior  lymph  hearts  of  the  amphibia 
bud  off  from  the  vetebral  veins  (Hoyer  49),  or  in  a  more  primitive 
position  from  branches  of  the  earlier  segmental  veins  which  are  going 
to  form  the  vertebrals  (Knower  74).  They  develop  earlier  than  the 
posterior  hearts  and  have  been  found  in  R.  palustris,  R.  sylvatica  and 
R.  virescens  measuring  about  6.5  mm.,  and  in  R.  temporaria  and 
Bufo  vulgaris  measuring  about  4.5  mm.  Hoyer  noted  the  presence 
of  blood  in  the  early  stages  of  both  anterior  and  posterior  hearts. 
Knower  brings  out  the  fact  in  connection  with  the  anterior  hearts 
that  they  lie  on  the  myotomes  from  which  they  derive  the  striated 
muscle  of  their  wall.  This  is,  I  think,  a  very  important  point.  The 
particular  myotome  Knower  has  shown  may  vary. 

The  pulsations  of  the  anterior  lymph  hearts  can  be  readily  seen  in 
the  dorsal  surface  of  the  tadpole,  just  behind  the  pronephros,  and  thus 
they  can  be  injected. 

From  the  anterior  lymph  heart  develop  two  symmetrical  vessels, 
one  of  which  runs  forward  toward  the  head,  the  other  backward  toward 
the  tail.  The  posterior  duct  as  described  by  Hoyer  divides  into  two 
branches,  one  running  to  the  tip  of  the  tail  dorsal  to  the  myotomes, 
the v  other  passing  back  along  the  ventral  margins  of  the  myotomes 
to  form  the  ventral  caudal  trunk.  It  is  the  branches  of  these  two 
caudal  trunks  which  have  been  studied  so  much  in  the  living  specimen. 
There  is  also  a  lateral  branch  which  runs  caudalward  from  the  an- 
terior lymph  hearts  on  the  lateral  surface  of  the  myotomes  half  way 
between  the  dorsal  and  ventral  branches.  This  lateral  trunk  subse- 
quently connects  with  the  posterior  lymph  heart. 

Wieliky  (153),  Jossifov  (62-65),  and  Favaro  (36)  thought  that  the 
posterior  lymph  heart  arose  from  the  dilation  of  the  caudal  lymph 
trunks  which  grow  from  the  anterior  lymph  hearts,  and  Jourdain 
(61)  describes  them  as  being  formed  by  a  rapid  destruction  of  con- 


The  Origin  and  Development  of  the  Lymphatic  System.       57 

nective  tissue.  Knower  and  Hoyer,  however,  have  found  that  they 
bud  off  from  the  posterior  vertebral  veins  considerably  later  than  the 
anterior  lymph  hearts,  and  they  have  been  well  worked  out  by 
Baranski  (9).  Baraiiski  shows  that  they  arise  by  several  buds  from 
the  endothelium  of  the  posterior  vertebral  vein  and  its  branches.  His 
fig.  1  shows  particularly  well  the  heaping  up  of  the  endothelium  so 
that  it  looks  like  a  solid  mass  at  the  point  of  origin.  Practically 
nothing  is  known  of  the  origin  of  the  deep  lymphatics  in  amphibia. 

It  has  thus  been  shown  that  the  lymph  hearts  of  amphibia  arise  as 
buds  or  sacs  from  the  endothelial  wall  of  segmental  veins.  They  lie 
on  the  myotomes,  from  which  they  derive  the  striated  muscle  of  their 
wall. 

The  subcutaneous  lymph  sacs  of  the  anura  are  secondary  structures. 
This  was  found  out  by  Eanvier  (116)  in  1896,  and  also  by  Knower 
and  Hoyer.  They  are  derived  from  ducts  which  grow  from  the 
lymph  hearts.  They  have  been  comparatively  little  studied.  Hoyer 
(49)  found  that  the  large  sac  on  the  ventral  surface  of  the  head  (sacc. 
submaxillaris)  developed  from  a  branch  of  a  duct  from  the  anterior 
lymph  heart,  and  that  the  lateral  sacs  are  derived  from  the  lateral 
lymph  trunks. 

This  point  has  been  just  shown  in  connection  with  the  sacs  of  the 
extremities  by  Frl.  Goldfinger  (38),  who  has  injected  the  primary 
ducts  and  then  the  increasingly  denser  lymphatic  plexus  up  to  the  final 
lymph  sacs.  This  process  of  cavernization  could  be  studied  here  by 
silver  nitrate  injections. 

Since  the  above  was  written  Hoyer  (49a)  has  published  an  article 
in  which  he  gives  a  general  review  of  the  origin  of  the  lymphatic 
system  in  vertebrates,  and  Hoyer  and  Udziela  have  given  the  first 
comprehensive  description  of  the  lymphatic  system  in  a  urodele  (49b). 
In  a  salamander  larva  the  lymphatic  system  opens  into  the  veins 
first  through  symmetrical  axillary  lymph  sacs  or  sinuses,  and  secondly 
through  a  series  of  subcutaneous  lymph  hearts  which  pump  the  lymph 
into  the  vena  lateralis  (Meyer  88a  and  Marcus  87a). 

There  are  six  longitudinal  lymph  trunks,  four  superficial  and  two 
deep  (Hoyer  49b,  Taf.  XII,  Figs.  1,  2  and  5).  (1)  The  median,  dor- 
sal, longitudinal  lymph  trunk,  which  extends  from  the  point  of  the  tail 
to  the  middle  of  the  head;  (2)  a  medial,  ventral  caudal  trunk,  which 
branches  around  the  cloaca  and  opens  into  the  inguinal  sinus  as  is 
shown  in  fig.  2  (49b) ;  (3  and  4)  symmetrical  lateral  trunks,  which 


58  Florence  R.  Sabin. 

lie  external  to  the  myotomes  and  open  in  part  into  the  segmental 
lymph  hearts  and  in  part  into  the  axillary  sinus,  and  (5  and  6)  the 
deep  subvertebral  lymph  trunks.  The  subvertebral  lymph  trunks  are 
paired  vessels  with-  many  anastomoses  which  extend  from  the  tip  of 
the  tail  to  a  point  opposite  the  stomach,  where  they  unite  to  form  a 
cisterna  chyli.  From  the  cisterna  chyli  four  vessels  run  forward  to 
the  axillary  plexus,  the  paired  thoracic  duct  and  the  two  paravertebral 
lymph  trunks. 

The  axillary  lymph  sacs  open  by  three  or  four  branches  into  the 
cardinal  veins  and  receive  the  following  vessels:  (1)  The  lateral 
superficial  lymph  trunks;  (2)  the  paravertebral  vessels;  (3)  the  thor- 
acic duct;  (4)  vessels  of.  the  cranial  part  of  the  stomach  and  esopha- 
gus; (5)  a  vessel  which  runs  in  the  groove  between  the  pericardial 
and  peritoneal  sacs;  (6)  a  vessel  which  comes  from  the  base  of  the 
skull;  (7)  superficial  vessels  from  the  side  of  the  head,  and  (8)  the 
lymph  vessels  of  the  forelegs. 

Besides  the  axillary  sac  there  are  two  other  sacs  or  sinuses,  one  of 
which  lies  dorsal  to  the  aorta  at  the  base  of  the  heart,  namely,  the  sinus 
lymphaticus  cordis  (Hoyer  49b,  fig.  5),  and  is  in  the  course  of  the  jug- 
ular lymph  trunks  and  the  other  of  which  makes  the  paired  sinus  of 
the  inguinal  region. 

The  segmental  lymph  hearts  receive  three  groups  of  vessels:  (1) 
branches  from  the  subvertebral  lymph  vessels  (which  agrees  with 
the  findings  of  Marcus  87a,  in  Gymnophionem ;  (2)  dorsal  and  ven- 
tral segmental  branches,  and  (3)  the  longitudinal  lymph  trunk. 
They  open  into  the  vena  lateralis.  The  segmental  lymph  hearts  and 
the  sinus  lymphaticus  cordis  have  striated  muscle  in  their  walls.  It 
is  clear  that  a  knowledge  of  the  origin  of  this  system  would  be  of 
great  value,  as  Hoyer  says :  "  Erst  wenn  die  beriihrten  noch  zweifel- 
haften  Punkte  durch  weitere  Untersuchungen  vervollstandigt  sein 
werden,  wird  eine  gewisse  Grundlage  zu  vergleichenden  Betrachtungen 
des  Lymphgefassystems  der  niederen  Wirbeltiere  geschaffen  sein " 
(49b,  p.  555). 

Stromsen  (145)  and  Huntington  (57)  have  worked  on  the  de- 
velopment of  the  lymphatic  system  in  reptiles.  Stromsen  shows  that 
the  posterior  lymph  hearts  develop  in  relation  to  the  coccygeal  veins. 
He  describes  the  process  as  a  combination  of  veins  and  dilated  spaces 
similar  to  Sala's  decription  for  the  corresponding  hearts  in  birds. 
This  method  of  formation  has  now  been  disproved  by  the  observation 


The  Origin  and  Development  of  the  Lymphatic  System.       25 

phritic  vein  and  the  veins  in  the  dorso-medial  edge  of  the  Wolffian 
bodies.  The  lymphatics  which  grow  from  the  ventral  surface  of  the 
mesonephritic  veins  make  the  large  retroperitoneal  or  prge-aortic  sac; 
those  which  grow  along  the  dorso-medial  edge  of  the  Wolffian  bodies, 
lateral  to  the  aorta,  form  the  iliae  sacs,  and  those  which  grow  dorsal 
to  the  aorta  make  the  cisterna  chyli  and  thoracic  duct.  The  thoracic 
duct  is  formed  in  small  part  from  a  duct  from  the  left  jugular  sac ; 
in  larger  part  from  a  plexus  which  surrounds  the  aorta  from  the 
cisterna  chyli.  The  retroperitoneal  sac  drains  the  diaphragm  and  the 
abdominal  viscera,  except  the  permanent  kidney;  the  iliac  sacs  drain 
the  permanent  kidney,  the  abdominal  walls,  the  hindlegs,  tail  and  em- 
bryonic membranes. 


B.    THE   JUGULAR   SACS. 

As  has  already  been  stated  in  part  III,  the  spreading  of  the  primary 
superficial  lymphatic  capillaries  in  the  skin  of  the  embryo  pig  takes 
place  while  the  embryo  is  growing  from  20  to  50  mm.  long.  The  first 
lymphatic  buds  are  found  when  the  embryo  is  about  11  mm.  long 
(fig.  6);  the  sac  is  well  formed  at  18  mm.  (fig.  7),  and  reaches  its 
maximum  size  in  an  embryo  30  mm.  long  (fig.  8).  It  remains  as  a 
large  sac  until  the  embryo  measures  50  mm.,  when  it  begins  to  be 
transformed  in  part  into  lymph  glands  (fig.  16). 

An  understanding  of  the  development  of  the  jugular  sac  depends 
on  a  knowledge  of  the  veins  of  the  region.  Indeed,  I  was  not  able  to 
inject  the  early  stages  until  I  had  so  mastered  the  pattern  of  the 
blood  vessels  that  a  little  blood  in  any  of  them  enabled  me  to  repro- 
duce the  whole  picture  and  so  avoid  the  blood  vessels. 

The  study  of  the  form  of  the  early  anterior  cardinal  vein  is  shown 
for  the  chick  by  Evans  (34)  in  his  figs.  1  to  3.  These  figures  show 
that  the  primitive  deep  dorsal  segmental  branches  drain  the  ventro- 
lateral  surface  of  the  spinal  cord.  They  bring  out  the  gradual  curve 
of  the  anterior  cardinal  vein  and  the  more  marked  bend  of  the  pos- 
terior cardinal  vein,  where  the  two  join  the  duct  of  Cuvier.  This 
stage  is  shown  for  the  chick  in  cross  section  by  Evans  (35)  in  his 
fig.  437  B,  as  well  as  in  (Evans  34)  his  fig.  4. 

In  fig.  4  is  given  an  injection  of  the  blood  vessels  in  an  embryo  pig 
7  mm.  long,  a  stage  just  before  the  lymphatics  begin.  The  curve  of 
the  dorsal  border  of  the  cardinal  veins  toward  the  duct  of  Cuvier  is 


Florence  E. 


slight,  not  as  marked  as  in  the  chick,  nor  as  it  is  on  the  left  side  of  this 
same  embryo. 

Arising  from  the  cardinal  veins  there  is  a  series  of  seven  deep 
segmental  branches  extending  to  the  nervous  system  betAveen  the 
area  of  the  primitive  posterior  cerebral  vein  (Mall  143),  which  arches 


FIG.  4. — Injected  specimen  to  show  the  anterior  cardinal  vein  and  its 
branches  in  an  embryo  pig  which  measured  7  mm.  in  its  greatest  diameter. 
Magnified  about  28  times.  This  is  a  stage  before  the  lymphatic  vessels 
have  begun.  A  =  aorta  arches;  D.  C.  =  duct  of  Cuvier;  V.  c.  p.  =  vena 
cerebralis  posterior;  V.  5  =  fifth  segmental  vein. 

downward  over  the  medulla  and  cord  and  the  cerebral  end  of  the  arm 
bud.  The  first  two  of  these  branches  drain  the  medulla,  and  hence 
there  are  five  spinal  or  segmental  branches  proper.  Each  of  these 
branches  has  a  corresponding  artery  which  lies  slightly  caudalward 
and  nearer  the  midline.  Only  the  lower  four  segmental  arteries  are 
injected  in  this  specimen.  The  relative  position  of  artery  and  vein 


The  Origin  and  Development  of  the  Lymphatic  System.       31 

budding  out,  though  the  internal  jugular  vein  as  a  whole  is  formed 
considerably  later. 

For  the  present  the  interest  in  the  superficial  blood  capillary  plexus 
is  that  it  covers  the  lymphatic  ridge.  It  will  be  seen  in  fig.  6  that  the 
lymphatic  ridge  includes  a  non-vascular  area.  This  non-vascular  area 
is  bounded  by  the  anterior  cardinal  vein  and  its  dorsal  branches,  the 
plexus  of  the  groove,  the  wide-meshed  superficial  plexus  and  the 
ventro-lateral  branches  of  the  anterior  cardinal  vein.  An  occasional 
direct  lateral  branch  of  the  cardinal  vein  cuts  through  the  otherwise 
non-vascular  area.  Within  this  area  are  seen  the  blood-filled  lymphatic 
buds.  They  lie  in  the  angle  between  the  anterior  cardinal  vein  and  its 
dorso-lateral  branches  and  connect  with  both.  They  are  opposite  the 
third,  fourth  and  fifth  dorsal  segmental  veins,  and  extend  a  distance 
of  1.5  mm.  from  the  primitive  ulnar  along  the  anterior  cardinal  vein. 
The  tiny  lymphatic  buds  are  already  sending  sprouts  away  from  the 
parent  vein. 

The  presence  of  stagnant  blood,  which  has  proved  so  valuable  a 
criterion  in  the  living  embryo,  must  be  used  guardedly  in  studying 
sections,  unless  there  is  a  complete  vascular  injection.  In  the  lym- 
phatic area  I  have  noted  in  the  fresh  embryo  that  the  blood  capillary 
plexus  of  the  groove  is  often  empty,  while  the  superficial  plexus  over 
the  lymphatic  area  and  over  the  pericardium  tend  to  retain  some 
blood.  The  blood  vessels  of  an  area  where  lymphatics  are  budding 
must  be  thoroughly  known  before  one  can  be  sure  of  the  lymphatic 
buds.  The  early  lymphatic  buds  are  packed  with  blood  to  an  extent 
not  common  for  the  veins,  which  perhaps  shows  best  in  figs.  490  and 
491  (Sabin  134),  for  the  jugular  lymphatic  buds  in  a  human  embryo. 

I  have  not  yet  succeeded  in  injecting  the  first  jugular  lymphatic 
buds  in  the  pig  as  E.  L.  Clark  has  clone  for  the  early  stages  of  the 
posterior  lymph  heart  of  the  chick.  In  the  embryo  12  mm.  long  they 
can  be  seen  in  the  fresh  embryo,  and  sections  show  that  they  have 
formed  a  plexus  along  the  margin  of  the  anterior  cardinal  vein.  To 
see  the  lymphatic  buds  at  12  mm.  the  embryo  should  be  placed  in  warm 
Locke's  solution  while  the  heart  is  still  beating,  and  it  should  be  viewed 
under  the  high  powers  of  the  binocular  microscope  with  the  direct 
sunlight  focused  upon  it.  The  lymphatic  area  is  wider  in  the  dorso- 
ventral  direction  than  in  fig.  6,  but  its  dorsal  boundary  is  clearly 
marked  by  the  surface  groove.  If  the  superficial  blood  capillary 
plexus  is  empty  the  lymphatics  can  then  be  seen  looking  like  a  cluster 


32  Florence  R.  Sabin. 

of  dull  red  grapes  against  the  cardinal  vein;  in  the  older  specimens 
some  of  the  buds  are  markedly  larger  than  others,  while  at  14  mm. 
the  largest  of  them  makes  a  definite  sac  with  large  sprouts  projecting 
dorsalward.  At  this  stage,  namely,  14  mm.,  the  sac  can  be  injected 


FIG.  7. — Injection  of  the  left  jugular  lymph  sac  in  an  embryo  pig  which 
measured  18  mm.  in  its  greatest  diameter.  The  specimen  was  fixed  in  10 
per  cent  formol  and  cleared  by  the  Spalteholz  method.  Magnified  about  30 
times.  The  glass  canula  which  shows  piercing  the  upper  border  of  the  arm 
bud  shows  the  point  of  injection.  A.  c.  —  anterior  curvature  of  the  lymph 
sac;  A.  s.  =  apex  of  the  lymph  sac;  I.  =  glass  canula  used  for  injection; 
S.  s.  =  stalk  of  the  lymph  sac;  V.  j.  i.  =  vena  jugular  interna. 

by  direct  puncture.  The  preceding  or  plexus  stage  is  shown  by  E.  L. 
Clark  for  the  corresponding  sac  in  the  chick  in  fig.  15.  The  beginning 
sac  is  shown  for  the  human  embryo  in  fig.  491  (Sabin  134). 

It  is  not  easy  to  get  perfect  injections  by  direct  puncture,  but  by 
the  time  the  embryo  is  1G  mm.  the  dorsal  sprouts  from  the  sac  are 


The  Origin  and  Development  of  the  Lymphatic  System.       63 

long  have  been  injected  from  one  puncture.  This  is  a  complete  injec- 
tion, which  it  is  clear  that  fig.  16  is  not.  In  fig.  5  (Sabin  130)  for 
example  the  lymphatics  between  the  eye  and  ear  are  terminal  ducts. 

Good  examples  of  the  anastomoses  of  different  groups  of  lymphatics 
are  shown  in  fig.  16  in  the  vessel  which  connects  the  submaxillary 
and  the  facial  plexus;  moreover  the  thoracic,  cervical  and  brachial 
plexuses  are  continuous. 

In  the  embryo  pig  from  5  to  5.5  cm.  long  there  are  no  valves  in  the 
ducts.  It  is  also  the  stage  of  the  simple  primary  plexus  of  lymphatics. 
In  fig.  16  a  secondary  plexus  of  finer  lymphatics  is  growing  more 
superficially  and  by  the  time  the  embryo  is  from  6  to  7  cm.  long 
there  is  a  deep  plexus  with  valves  and  a  finer-meshed  superficial  plexus. 

The  relation  of  the  jugular  lymph  sacs  to  lymph  glands  is  shown  in 
fig.  17.  Here  it  is  clear  that  the  sac  stalk  and  anterior  curvature  lie 
beneath  the  sternocleidomastoid  muscle  and  that  the  apex  of  the 
sac  lies  in  the  posterior  triangle  of  the  neck.  That  the  cervical  plexus 
is  now  a  lymph  gland  is  clear,  and  there  is  a  facial  and  submaxillary 
gland  also.  The  cervical  plexus  becomes  a  group  of  glands. 

The  account  of  the  peripheral  lymphatics  from  the  jugular  sacs  is 
not  complete  without  mention'  of  the  lymphatics  of  the  heart  and  lungs. 
These  lymphatics  have  not  yet  been  worked  out,  but  it  can  be  said  that 
there  are  two  sets  of  lymphatics  for  the  lungs,  the  deep  and  the  super- 
ficial or  pleural.  The  deep  lymphatics  develop  from  the  jugular  sacs; 
they  follow  the  trachea  and  are  present  in  a  pig  measuring  23  mm. 
and  a  human  embryo  measuring  20  mm.  I  have  not  injected  them  and 
when  reconstructed  they  show  the  Mayer-Lewis  anlagen;  that  is,  in 
reconstruction  they  split  into  a  chain  of  beads.  The  pleural  lymphatics 
I  have  injected  from  the  retroperitoneal  sac,  through  the  diaphragm 
to  the  caudal  surface  of  the  lung.  Injections  of  the  thoracic  duct  as, 
for  example,  fig.  7  (Nuck  100),  also  shows  vessels  from  the  thoracic  duct 
to  the  lung.  Therefore  the  lung  seems  to  have  a  double  supply  of  lym- 
phatics, part  from  the  jugular  sac  and  part  from  the  renal  sacs  by 
way  of  the  diaphragm  and  by  way  of  the  thoracic  duct. 


B.    THE   PERIPHERAL   LYMPHATICS   FROM  THE  RENAL  SACS. 

The  retroperitoneal  sacs  give  rise  to  the  lymphatics  of  the  abdominal 
viscera  except  the  kidneys.  The  sac  as  it  lies  in  the  root  of  the  mesen- 
tery has  been  figured  by  Heuer  (43).  It  spreads  over  the  ventral 


64  Florence  R.  Sabin. 

surface  of  the  Wolffian  bodies  and  supplies  their  capsule  as  well  as 
the  reproductive  glands.  From  the  sac  vessels  grow  to  the  stomach 
and  the  entire  intestine,  including  the  rectum.  The  lymphatics  of 
the  diaphragm  can  be  injected  from  the  retroperitoneal  sac.  The  de- 
velopment of  the  lymphatics  of  the  diaphragm,  liver,  capsule  of  the 
spleen,  adrenal  and  pancreas  has  not  yet  been  worked  out. 

Heuer  (43)  has  followed  the  lymphatics  of  the  intestine  from  the 
retroperitoneal  sac  to  the  ultimate  lacteals.  While  the  embryo  is 
growing  from  3  to  4  cm.  long  a  very  abundant  plexus  of  capillaries 
spreads  out  in  the  mesentery.  The  lymphatics  reach  the  wall  of  the 
intestine  by  the  time  the  embryo  is  4  cm.  long.  They  first  enter  the 
submucosa  and  form  then  a  primary  plexus.  Heuer's  figures  9  and 
10  show  that  the  early  vessels  in  the  submucosa  have  a  segmental 
arrangement,  which  is,  however,  nearly  lost  as  the  complete  plexus 
forms.  The  plexus  of  the  mucosa,  and  of  the  serosa  are  both  second- 
ary. The  lacteals  develop  from  the  mucosal  plexus  and  are  present 
in  a  pig  measuring  9  cm.  The  mesenteric  vessels  and  the  submucosal 
plexus  develop  valves. 

The  iliac  lymphatics  and  the  thoracic  duct :  The  lymphatics  which 
arise  from  the  mesonephritic  veins  on  the  two  sides  form  a  very 
simple  pattern.  Starting  from  the  veins  at  the  hilus  of  the  two 
Wolffian  bodies  they  grow  caudalward  along  the  edge  of  the  Wolffian 
bodies  to  make  the  iliac  sacs,  and  cerebralward  along  the  dorsal  sur- 
face of  the  aorta  to  form  the  cisterna  chyli.  The  lymphatics  dorsal 
to  the  aorta  grow  in  two  directions:  (1)  caudalward  to  form  a  chain 
of  praevertebral  lymph  nodes  and  (2)  cerebralward  to  form  the  thoracic 
duct. 

The  iliac  lymphatic  sacs  are  two  long  symmetrical  sacs  extending 
from  the  hilus  of  the  Wolffian  bodies  to  the  level  of  the  bifurcation 
of  the  aorta.  In  an  embryo  23  mm.  long  they  drain  into  the  mesone- 
phritic veins.  By  the  time  theembryo  is  25  to  27  mm.  long  this 
connection  is  lost  and  the  two  iliac  sacs  converge  into  the  median 
cisterna  chyli  dorsal  to  the  aorta. 

The  caudal  end  of  the  iliac  lymphatics  in  the  pig  is  a  sac  of  con- 
siderable size,  from  which  three  sets  of  lymphatics  can  be  injected: 
the  ilio-lumbar,  the  femoral  and  a  plexus  which  surround  the  umbil- 
ical arteries.  The  ilio-lumbar  superficial  lymphatics  are  very  con- 
spicuous in  the  pig,  as  shown  in  figs.  4  and  5  (Sabin  130).  This  is  in 
marked  contrast  to  the  human  embryo,  where  they  form  part  of  the 
inguinal  group. 


The  Origin  and  Development  of  the  Lymphatic  System.       65 

2.  PERIPHERAL  LYMPHATICS  IN  OTHER  FORMS. 

The  pattern  of  the  superficial  lymphatics  in  the  human  embryo  is 
shown  in  the  two  figs.,  505  and  506  (Sabin  134).  This  was  a  re- 
markable specimen  of  an  embryo  5.5  cm.  long  which  had  been  kept  in 
formalin  some  time.  On  transferring  it  to  freshly  made  up  alcohol, 
air  filled  the  entire  superficial  lymphatic  system.  It  shows  the  primary 
superficial  lymphatic  plexus.  Valves  in  a  few  ducts,  namely,  the 
occipital,  the  thoracic  and  the  inguinal,  indicate  the  oldest  vessels. 
The  differences  between  the  superficial  pattern  in  the  human  and  in  the 
pig  are  mainly  the  fact  that  in  the  human  embryo  the  thoracic  lym- 
phatics are  much  more  prominent,  draining  into  the  axilla,  while  in  the 
pig  they  drain  into  the  superficial  cervical  plexus,  and  all  the  posterior 
lymphatics  drain  into  the  inguinal  region  in  the  human  embryo.  In 
the  human  embryo  the  deep  lymphatics  for  the  arm  grow  from  an 
extension  of  the  jugular  sac  along  the  primitive  ulnar  veins,  and  the 
deep  lymphatics  for  the  legs  grow  from  the  iliac  sac  along  the  femoral 
veins.  This  shows  in  fig.  493  (Sabin  134). 

Babbit  and  cat  embryos  are  like  the  human  in  having  a  marked 
development  of  the  jugular  sac  along  the  primitive  ulnar  vein  (Lewis 
76).  In  this  respect  they  differ  from  the  pig  where  the  axillary  lym- 
phatics are  smaller.  In  fig.  7  is  shown  a  deep  vessel  from  the  jugular 
sac,  which  is,  I  think,  the  axillary  duct  in  the  pig.  In  the  rabbit  and 
the  cat  the  axillary  sac  may  possibly  arise  independently  of  the  jugular 
sac,  and  in  the  rabbit  a  conspicuous  chain  of  peripheral  vessels  grows 
along  the  thoraco-epigastric  vein  (fig.  8,  Lewis  76).  Injections  of  cat 
embryos  show  that  the  axillary  lymphatic  trunk  is  very  large. 

The  superficial  pattern  of  the  lymphatics  in  bovine  embryos,  as 
shown  by  Polinski  (107),  is  much  like  that  of  the  pig. 

In  the  chick  the  primary  lymphatic  vessels  in  the  skin  connect 
the  posterior  lymph  heart  with  the  jugular  lymph  sac  along  the  lateral 
line  in  the  course  of  the  thoraco-epigastric  vein  Mierzewski  (96) 
and  E.  L.  Clark  (27  and  29). 

VI.  ORIGIN  AND  DEVELOPMENT  OF  LYMPHATIC  GLANDS 
AND  THEIR  RELATION  TO  PRIMARY  LYMPH  SACS. 

It  is  easy  to  prove  that  the  lymphatic  sacs  in  mammals  and  the 
jugular  lymph  sac  in  birds  become  in  part  lymph  glands.  I  shall  define 
primary  lymph  glands  as  those  which  are  derived  from  the  primary 
lymph  sacs. 


66  Florence  R.  Sabin. 

From  the  jugular  lymph  sac  in  the  pig  it  is  clear  in  fig.  16  that 
two  lymph  glands  develop:  (1)  the  deep  jugular  lymph  node  which 
drains  the  pharynx  and  (2)  the  node  in  the  posterior  triangle  of  the 
neck  which  drains  the  skin  of  the  anterior  part  of  the  body.  A.  H. 
Clark  (24)  has  shown  that  these  two  glands  are  single  glands  in  the 
neck  of  the  adult  pig  and  that  they  become  the  largest  of  all  the  cervical 
glands.  In  the  human  embryo  these  two  glands  are  represented  by 
groups  of  glands,  the  glands  of  the  posterior  triangle  and  the  deep 
jugular  glands.  In  the  human  embryo  also  the  group  of  deep  axillary 
glands  comes  from  an  extension  of  the  jugular  sac  along  the  primitive 
ulnar  vein  and  hence  they  are  primary  lymph  glands.  Thus  the  pri- 
mary nodes  for  the  anterior  part  of  the  body  are  (1)  deep  jugular  nodes, 

(2)  the  ngdes  of  the  posterior  triangle,  (3)  the  axillary  nodes.    The 
relation  of  the  primary  lymph  gland,  both  to  the  various  structures 
of  the  neck  and  to  the  stalk  of  the  jugular  sac,  that  is,  to  the  jugular 
lymph  trunks,  is  shown  especially  well  in  fig.  17  from  pig  7.5  cm. 
long.     The  anterior  curvature  and  the  sac  stalk  lie  behind,  that  is, 
medial  to  the  sternocleidomastoid  muscle.     Of  the  primary  nodes  in 
the  neck  the  deep  jugular  nodes  drain  a  restricted  area,  namely,  the 
pharynx  and  nose;  the  axillary  lymph  nodes  drain  the  arm,  and  the 
large  node  of  the  posterior  triangle  drains  all  the  rest  of  the  head,  face, 
neck  and  thorax,  either  directly  or  through  the  superficial  cervical 
plexus.    The  nodes  of  the  superficial  cervical  plexus  are  secondary  to 
the  primary  lymph  sac  nodes.     It  has  been  brought  out  that  these 
groups  of  lymphatics  do  not  remain  distinct  as  they  arise,  but  all  be- 
come connected  with  each  other  through  anastomoses  of  the  lymph 
vessels. 

From  the  retroperitoneal  sac  develop  the  retroperitoneal  lymph 
nodes.  From  the  iliac  sacs  in  the  pig  there  is  a  chain  of  small  nodes 
lateral  to  the  aorta  and  a  large  group  of  glands  on  either  side  opposite 
the  bifurcation  of  the  aorta.  These  are  primary  iliac  nodes.  Dorsal 
to  the  aorta  is  a  chain  of  nodes  from  the  lower  end  of  the  cisterna 
chyli  to  the  bifurcation  of  the  aorta.  Thus  the  renal  lymphatics  give 
rise  to  three  groups  of  primary  lymphatic  glands:  (1)  pra-aortic  or 
retroperitoneal,  (2)  symmetrical  iliac  nodes  lateral  to  the  aorta,  and 

(3)  pra3- vertebral  nodes  dorsal  to  the  aorta. 

The  secondary  lymphatic  nodes  develop  along  the  lymphatic  vessels. 
The  most  extensive  group  of  secondary  nodes  in  the  embryo  pig  is  the 
one  which  comes  from  the  superficial  cervical  plexus  along  the  external 


The  Origin  and  Development  of  the  Lymphatic  System.       67 

jugular  vein.  In  fig.  17  this  plexus  is  shown  as  one  large  lymph 
gland.  From  the  cerebral  end  of  the  plexus  vessels  are  forming  a 
gland  at  the  point  where  the  lymphatics  divide  to  form  the  facial  and 
the  temporal  lymphatics.  The  cervical  plexus  in  the  adult  pig  becomes 
a  group  of  at  least  a  dozen  small  lymph  glands.  In  both  figs.  17  and 
18  there  is  a  developing  submaxillary  lymph  gland  along  the  course 
of  the  facial  branch  of  the  linguo-facial  vein  (fig.  8) .  The  small  facial 
node  and  the  submaxillary  node  are  tertiary  nodes  compared  with  the 
jugular  lymph  sac.  They  show  how  lymph  glands  develop  at  points 
where  lymph  ducts  radiate  out  on  their  development.  The  primary, 
secondary  and  tertiary  nodes  are  constant  in  the  pig. 

The  mesenteric  glands  are  secondary  for  the  retroperitoneal  sac. 
The  secondary  glands  from  the  iliac  sac  are  very  simple,  for  there  are 
only  two  of  them:  (1)  The  ileo-inguinal  gland,  which  is  very  charac- 
tesristic  of  the  pig  and  which  remains  as  a  single  gland.  Its  position 
is  readily  made  out  in  fig.  4  (Sabin  130),  for  the  superficial  lymphatics 
of  the  posterior  body  Avail  radiate  to  it.  Like  the  superficial  cervical 
glands  it  is  superficial.  (2)  The  inguinal  glands  in  the  pig  become 
a  large  group  of  superficial  glands,  the  position  of  which  can  be  made 
out  in  fig.  5  (Sabin  130).  The  lymph  ducts  from  the  inguinal  lymph 
glands  develop  to  the  leg,  the  ventral  abdominal  wall  and  the  external 
genital  organs.  The  efferent  vessels  of  the  inguinal  group  of  glands 
and  ileo-inguinal  gland  converge  to  the  large  group  of  primary  iliac 
glands  opposite  the  bifurcation  of  the  aorta. 

The  histological  development  of  lymph  glands  is  now  well  under- 
stood. The  work  of  Saxer  (138)  on- developing  lymph  glands  considers 
primarily  the  development  of  lymphocytes  which  I  shall  not  attempt 
to  discuss  in  this  paper,  confining  the  work  to  the  development  of  the 
gland  as  a  whole.  From  the  time  of  Breschet  (16)  it  has  been  known 
that  lymph  glands  begin  as  a  plexus  of  lymphatic  ducts.  Their  de- 
velopment can  be  followed  through  the  work  of  Eanvier  (117),  Bartels 
(10),  Gulland  (39),  Kling  (72),  Jolly  (59),  Lewis  (77),  and  Sabin 
(131). 

The  primary  lymph  glands,  which,  with  the  exception  of  the  post- 
aortic,  develop  out  of  sacs,  begin  by  a  bridging  of  the  sac  by  bands  of 
connective  tissue  covered  by  endothelium.  In  the  case  of  the  retroperi- 
toneal sac  these  bands  are  never  absent  (figs.  498  and  499,  Sabin  134). 
For  the  jugular  lymph  sac  they  show  well  in  human  embryo  30  mm. 
long  in  fig.  495  (Sabin  134).  That  these  bridges  are  entirely  covered 


68  Florence  R.  Sabin. 

by  endothelium  is  best  brought  out  by  total  mounts  of  silver  nitrate  in- 
jections. This  has  been  done  for  the  retroperitoneal  sac  in  the  pig. 
When  the  primary  sacs  are  thus  completely  bridged  by  these  bands 
they  are  practically  a  dense  plexus  of  lymphatic  capillaries  and  are 
therefore  in  the  first  stage  of  the  development  of  lymph  glands.  At 
this  stage  the  connective  tissue  septa  are  undifferentiated  and  contain 
only  mesenchyme  and  blood  capillaries. 

We  are  now  in  a  position  to  consider  the  question  of  which  are  the 
first  lymph  nodes  in  the  embryo.  This  point  has  been  discussed  by 
Lewis  (77)  and  Sabin  (131  and  134).  It  depends  on  the  criterion 
used.  If  the  criterion  is  recognizing  the  formation  of  a  plexus  of 
lymphatic  capillaries  with  undifferentiated  connective  tissue  bridges 
as  the  anlagen  of  a  gland,  then  it  is  clear  that  the  primary  lymph 
glands,  that  is,  those  that  come  from  the  lymph  sacs,  are  the  first 
lymph  glands  to  begin  for  each  region  of  the  body.  Thus  the  jugular- 
subclavian  sac  in  the  human  embryo  is  extensively  bridged  at  30  mm. 
and  is  therefore  a  plexus  of  lymphatics,  the  anlage  of  a  lymph  gland. 
At  this  stage  we  are  agreed  there  are  no  lymphocytes  (Kling  (72), 
/Lewis  (77)  and  Sabin  (131)).  If,  on  the  other  hand,  the  criterion 
is  the  development  of  the  first  lymphocytes  in  the  body,  we  must 
say  that  this  point  has  not  been  reinvestigated  since  all  the  primary 
lymph  nodes  of  the  embryo  have  been  determined.  Saxer  (138, 
p.  381)  notes  that  lymph  nodes  are  beginning  in  the  neck,  the  pos- 
terior mediastinum,  the  retroperitoneal  tissue,  the  outside  of  both 
hips  and  along  the  Wolffian  bodies  in  bovine  embryos  4.5  cm.  long, 
and  in  sheep  embryos  of  the  same  length  describes  undoubted  wander- 
ing cells  in  the  same  areas.  Kling  (72)  has  no  specimens  of  human 
embryo  between  31  mm.,  where,  he  says,  there  are  no  lymphocytes  in 
the  axillary  glands,  and  70  m.,  when  lymphocytes  are  present  in  many 
glands.  Lewis  (77)  finds  lymphocytes  in  human  embryos  42  mm. 
long.  He  finds  that  they  occur  practically  simultaneously  in  the 
glands  around  the  internal  jugular  vein  (primary  lymph  nodes) 
and  certain  "  isolated  subcutaneous  lymph  glands,"  of  which  he  figures 
one  or  two  along  the  linguo-facial  vein  and  its  branches.  The  relation 
of  these  glands  to  lymphatic  vessels  will  be  readily  made  out  by  a 
comparison  of  fig.  8  and  fig.  16.  Figure  8  shows  the  ducts  from  the 
sac  along  the  linguo-facial  vein  at  a  stage  even  younger  than  Lewis's 
isolated  glands,  namely,  a  pig  measuring  3.5  cm.  Moreover  the  be- 
ginning cervical  plexus  from  which  these  subm axillary  vessels  come 


The  Origin  and  Development  of  the  Lymphatic  System.       69 

shows  plainly  in  fig.  7  from  an  embryo  much  smaller,  namely,  one 
measuring  18  mm.,  and  they  are  readily  injected  in  embryos  30  mm. 
long,  fig.  2  (Sabin  130).  Thus  it  is  almost  certain  that  the  ducts  for 
Lewis's  early  facial  glands  in  human  embryos  were  present,  but  could 
not  be  found,  in  sections.  It  is  probable  that  the  lymphocytes  begin 
in  human  embryos  which  measure  about  40  mm.  and  they  probably 
develop  almost  simultaneously  in  the  primary  and  in  the  secondary 
glands. 

The  plexus  stage  of  developing  lymph  glands  has  been  shown  by 
reconstruction  by  Kling  (72)  and  Lewis  (77),  and  by  injection  in 
fig.  10  (Sabin  131). 

The  whole  question  of  the  structure  of  a  lymph  gland  may  be 
summed  up  in  a  word  by  a  quotation  from  Eanvier  (p.  1038  in  117)  : 
"  Un  ganglion  lymphatique  est  un  angiome  caverneux  lymphatique  qui 
a  ete  d'abord  angiome  simple."  The  development  of  a  gland  involves, 
however,  two  processes :  ( 1 )  the  formation  of  lymph  sinuses  out  of 
lymph  plexuses,  and  (2)  the  formation  of  lymph  cords  and  lymph  folli- 
cles in  the  trabeculag.  A  comparison  of  Jolly's  (59)  plates  from  lymph 
glands  in  birds,  Kling's  (72)  figures  of  human  lymph  glands,  and 
my  own  (131)  from  the  developing  glands  in  the  pig,  will  show  that 
the  relative  proportion  of  these  two  parts  varies  exceedingly  in  differ- 
ent glands. 

The  formation  of  lymph  sinuses  can  be  best  understood  by  beginning 
with  Jolly's  figures.  He  shows  that  in  the  birds,  instead  of  the  mam- 
malian sinus,  there  is  a  diffuse  plexus  of  lymphatic  vessels  with  lym- 
phocytes in  the  septa  between  the  vessels.  He  also  brings  out  the 
fact  that  very  large  vessels  may  pass  directly  through  the  gland.  Thus 
it  is  clear  that  the  sinus  of  birds  is  a  simpler  structure  than  that 
of  mammals.  On  this  account  the  complete  covering  of  endothelium 
is  very  plain  in  Jolly's  figures.  Out  of  a  plexus  of  capillaries  such  as 
Jolly  shows,  the  mamalian  lymph  sinus  is  formed,  as  can  be  seen 
in  fig.  15  (Sabin  131),  by  an  increase  in  the  density  of  the  lymphatic 
plexus  until  the  septa  are  reduced  to  a  framework  of  reticulum  covered 
by  endothelium.  In  this  figure  all  the  stages  of  the  formation  of  a 
sinus  can  be  seen  in  a  single  section.  In  the  lower  part  of  the  specimen 
every  lymphatic  vessel  is  plain  with  its  complete  lining  of  endothelium, 
while  in  the  upper  part  the  septa  are  in  places  already  reduced  to  a 
line.  This  stage  is  from  an  embryo  pig  24.5  cm.  long,  which  is  the 


70  Florence  R.  Sabin. 

best,  stage  for  studying  the  formation  of  the  sinuses.     The  question 
of  the  development  of  reticulum  has  been  taken  up  by  Mall  (84). 

In  regard  to  the  formation  of  the  lymph  cords  and  follicles,  the  funda- 
mental point  is  the  question  of  the  origin  of  the  lymphocytes  themselves, 
this,  however,  I  shall  not  undertake  to  discuss.  The  lymphocytes  may 
*ccur  diffusely  in  all  of  the  trabeculse  of  the  developing  node  and  this 
4s  the  simplest  form  of  a  gland.  This  form  is  shown  by  Jolly  (59) 
in  his  plate  IX,  fig.  3,  for  the  lumbar  glands  in  birds,  by  Kling  (72), 
plate  XXVII,  fig.  11,  for  the  axillary  glands  in  human  embryos  and 
in  fig.  9  (Sabin  131)  from  the  primary  jugular  lymph  glands  in 
the  pig.  It  will  be  noted  that  all  of  these  figures  are  from  primary 
lymph  glands.  The  definite  follicles  may  develop  in  the  center  or  in 
the  periphery  of  a  node,  and  there  is  every  possible  variation  in 
the  proportion  of  the  diffuse  cords  and  the  definite  follicles.  There 
are  two  processes  in  the  development  of  the  follicle :  ( 1 )  an  increase 
in  the  number  of  lymphocytes  forming  a  definite  clump,  and  (2) 
the  formation  of  a  tuft  of  blood  capillaries.  Both  the  cords  and  the 
follicles  form  along  the  blood  vessels,  the  follicles  coming  at  the  cap- 
illary bed. 

As  I  have  said,  the  early  glands  which  come  from  the  primary  sacs 
pass  through  the  form  of  diffuse  distribution  of  the  lymphocytes. 
Follicles  which  develop  later,  however,  may  begin  at  once  as  clumps 
of  lymphocytes,  making  very  definite  follicles  surrounded  by  a  lym- 
phatic plexus.  A  number  of  such  tiny  follicles  are  to  be  seen  in  fig. 
16  (Sabin  131),  which  is  the  developing  group  of  inguinal  lymph 
glands  in  a  pig  24.5  cm.  long. 

Lymph  glands  may  be  either  simple,  consisting  of  one  follicle  with 
a  peripheral  sinus,  or  compound,  with  many  follicles  and  cords,  and 
both  peripheral  and  central  sinuses. 

The  subject  of  hg&molymph  glands  has  been  taken  up  by  Helly  in 
the  Ergebnisse  fur  Anatomic  and  Entwickelungsgeschichte  for  1902. 

VII.  COMPARATIVE  MORPHOLOGY  OF  THE  PRIMARY 
LYMPH  SACS,  LYMPH  HEARTS,  AMPHIBIAN  LYMPH 
SACS  AND  LYMPH  GLANDS. 

We  are  now  in  a  position  to  consider  the  comparative  morphology 
of  lymph  sacs  and  lymph  hearts.  I  shall  not  attempt  to  analyze  the 
work  of  Favaro  (36a)  and  of  Allen  (2-4)  on  lymphatics  in  fishes, 


The  Origin  and  Development  of  the  Lymphatic  System.       71 

both  of  whom  state  that  vessels  may  function  now  as  veins  and  now  as 
phatics,  but  beginning  with  the  amphibia,  it  is  certain  that  lym- 
phatics bud  off  from  the  veins  in  certain  definite  areas  and  form 
plexuses  which  are  transformed  into  sacs.  The  essential  structure  of 
these  sacs  is  the  endothelial  lining.  ^A  primary  lymph  sac  is  there- 
fore one  which  buds  off  from  the  endothelium  of  a  vein. 

In  the  amphibia,  probably  in  the  reptilia,  and  in  the  case  of  the 
posterior  lymph  sacs  in  birds,  the  primary  sacs  bud  off  from  segmental 
veins  and  rest  on  the  myotomes.  They  derive  striated  muscle  from 
the  myotomes,  which  has  been  shown  at  least  in  the  case  of  the 
anterior  lymph  hearts  in  amphibia  (Knower  74),  and  the  posterior 
lymph  hearts  in  reptiles  (Stromsen  145)  and  pulsate  rhythmically;  in 
a  word,  they  become  lymph  hearts. 

The  lymphatic  sacs  for  the  anterior  part  of  the  body  in  birds  and  all 
of  the  lymphatics  in  mammals  do  not  come  from  the  vertebral  and 
coccygeal  veins,  but  from  the  anterior  cardinal  veins  in  the  neck,  and 
the  renal  veins  in  the  abdomen.  These  sacs  do  not  lie  on  the  myotomes, 
they  do  not  receive  striated  muscle,  but  rather  are  transformed  into 
lymph  glands.  Thus  the  bird  represents  a  transition  stage  having  a 
posterior  lymph  heart  and  a  jugular  lymph  gland.  I  shall  keep, 
therefore,  the  term  anterior  and  posterior  lymph  hearts,  and  use  the 
terms  jugular  and  renal  lymph  sacs.  The  embryological  classification 
is: 

"1.  Lymph  hearts. 

a.  Anterior,  amphibia. 

b.  Posterior,  amphibia,  reptilia  and  birds. 
Primary 

Lymph  - 

a.  Jugular — birds  and  mammals. 

fRetroperitoneal. 

b.  Renal  J  Iliac. 

[cisterna  chyli. 

The  subcutaneous  and  deep  lymph  sacs  of  the  amphibia  are  not 
primary,  but  secondary.  They  are  transformed  lymphatic  ducts. 
Thus  the  amphibia  have  primary  lymph  sacs  which  become  lymph 
hearts,  lymph  ducts  which  become  secondary  sacs,  and  lymphatic  capil- 
laries. Mammals  have  primary  lymph  sacs  which  become  primary 
lymph  glands,  lymphatic  ducts  which  develop  valves  and  along  which 
secondary  glands  are  formed,  and  thirdly,  lymphatic  capillaries  without 
valves. / 


Sacs. 


7?  Florence  R.  Sabin. 

VITT.  VARIOUS  OTHER  THEORIES  IX  REGARD  TO  THE 
ORIGIX  AXD  DEVELOPMENT  OF  THE  LYMPHATIC 
SYSTEM. 

1.  MAYER-LEWIS  AXLAGEX. 

In  the  preceding  pages  has  been  given  a  consistent  account  of  how 
the  lymphatic  system  buds  off  from  the  veins  and  gradually  invades 
the  body.  The  places  where  the  lymphatics  bud  off  vary  in  different 
forms;  in  the  amphibia  and  reptiles,  and  in  the  case  of  the  posterior 
lymph  hearts  in  birds  the  primary  lymph  sacs  lie  in  the  myotomes  and 
receive  striated  muscle  and  become  lymph  hearts.  In  the  higher  forms 
the  anterior  lymphatics  come  from  the  anterior  cardinal  vein  and  its 
branches,  the  caudal  lymphatics  do  not  develop  (mammals)  and  the 


FIG.  18. — Diagram  to  show  the  theory  of  Ranvier  and  myself  that  the 
lymphatics  arise  from  the  veins.  The  veins  are  striped,  the  lymphatics 
dotted. 

posterior  lymphatics  come  from  the  vena  cava  and  renal  veins.  From 
these  points  of  origin  can  be  traced  all  the  lymphatics  of  the  body 
down  to  the  ultimate  capillaries. 

This  theory  has  not  been  developed  without  opposition.  In  section 
I  it  was  made  clear  that  the  first  theory  of  the  origin  of  the  lymphatics 
was  that  they  arose  in  the  periphery  as  dilated  tissue  spaces  and  grew 
towards  the  center,  Gulland  (39).  This  was  questioned  by  Sala 
(137),  showing  that  the  first  lymphatics  in  birds  were  against  the 
veins,  and  by  my  proving  in  1902  that  the  jugular  lymphatics  are 
the  first  lymphatics  in  mammals  and  that  they  bud  off  from  the 
anterior  cardinal  veins.  Thus  I  substituted  for  the  theories  shown  in 


The  Origin  and  Development  of  the  Lymphatic  System.       73 

figs,  1,  2  and  3  the  simple  hypothesis  of  Kanvier,  shown  in  fig.  18. 
This  indicates  that  lymphatics  bud  off  as  lymphatics  from  the  veins 
and  have  the  same  relation  to  tissue  spaces  as  have  blood  capillaries. 

The  most  difficult  obstacle  in  regard  to  this  theory  is  what  we  call 
Mayer-Lewis  anlagen.  Mayer  (89)  noted  in  the  tadpole's  tail  certain 
isolated  vessels  which  he  thought  were  evidences  of  the  degeneration 
of  blood  vessels. 

Ranvier  (114,  p.  578)  in  studying  the  plexus  of  lymphatics  of  the 
great  omentum.  in  a  new-born  cat  noted  that  a  great  many  of  the 
vessels  terminated  in  cul  de  sacs.  Some  were  attached  by  extremely 
slender  threads,  so  that  they  looked  as  if  they  were  becoming  iso- 
lated by  the  atrophy  of  the  intermediate  part.  Since  in  the  adult  cat 
there  are  no  lymphatics  in  the  omentum,  Banvier  interpreted  this  as 
a  method  of  degeneration  of  lymphatic  vessels. 

The  study  of  the  method  of  degeneration  of  blood  and  lymphatic 
capillaries  is  an  important  one.  It  is  obvious  that  there  must  be  a 
destruction  of  capillaries,  since  a  single  vessel  or  a  few  vessels  come 
from  a  plexus  of  capillaries.  In  Clark's  observations  (25,  p.  191)  the 
usual  method  of  degeneration  is  by  retraction  of  the  processes,  but 
he  has  also  observed  (26,  p.  410)  together  with  Mayer  and  Eanvier, 
that  an  occasional  segment  of  a  blood  capillary  or  lymph  capillary 
may  become  detached  and  be  left  to  atrophy.  This  is  in  accord  with 
some  of  the  findings  of  pathology.  We  must,  therefore,  conclude  thai 
detached,  degenerating  vessels  do  occur,  though  they  do  not  represent 
the  usual  method  of  degeneration. 

F.  T.  Lewis  (76)  found  that  in  reconstructions  lymphatics  looked 
like  chains  of  beads,  and  to  explain  this  phenomena  he  suggested  that 
lymphatics  might  grow  by  small  detached  portions  of  blood  vessels. 
These  hypothetical  lymphatic  anlagen  we  call  Lewis  anlagen. 

It  is  clear  in  watching  lymphatics  grow,  that  they  never  pick  up 
isolated  vessels,  but  grow  by  the  increase  in  the  protoplasm  of  their 
own  wall;  yet  it  is  clear  also  that  reconstructions  of  uninjected  lym- 
phatics appear  like  rows  of  beads. 

To  study  this  point  it  was  necessary  to  submit  the  method  of  recon- 
struction to  comparative  tests  with  the  method  of  injection  and  when 
possible  with  the  method  of  growth  in  the  living  form.  This  has  been 
done  three  times  (Clark,  E.  E.  26;  Clark,  E.  L.  27,  and  Sabin  135). 

The  best  opportunity  for  studying  this  point  is  the  tadpole's  tail, 
where,  in  the  living  specimen,  or  in  the  tail  fixed  in  alcohol,  the  entire 


74  Florence  R.  Sabin. 

lymphatic  system  can  be  seen  and  drawn.  This  was  done  by  Clark. 
The  specimen  was  then  cut  in  serial  sections  and  both  blood  capillaries 
and  lymphatics  were  reconstructed.  Two  reconstructions  were  made, 
one  with  the  4  mm.  Zeiss  objective,  and  the  other  with  a  2  mm.  Zeiss 
oil  immersion  lens.  Both  reconstructions  show  that  neither  blood 
capillaries  nor  lymphatic  capillaries  can  be  reconstructed  completely. 
More  is  obtained  with  an  oil  immersion  lens,  but  both  powers  show 
capillaries  in  the  form  of  rows  of  beads  (figs.  5,  6  and  7,  Clark  26, 
copied  as  figs.  513  and  514,  Sabin  134). 

This  test  of  Clark's  is  the  best  possible  test,  because  it  is  a  recon- 
struction of  exactly  the  same  specimen  from  which  the  original  draw- 
ing was  made.  This  point  cannot  be  made  in  testing  the  method  of  in- 
jection and  that  of  reconstruction.  For  this  test  I  ( 135 )  used,  however, 
symmetrical  plexuses  in  the  same  embryo.  An  embryo  pig  27  mm. 
was  chosen  in  which  there  was  an  almost  complete  injection  of  the  occip- 
ital superficial  lymphatics.  Many  of  the  sprouts  on  the  margin  had  been 
ruptured.  The  injected  plexus  could  of  course  be  reconstructed,  while  a 
reconstruction  of  the  empty  lymphatics  on  the  opposite  side  showed  the 
entire  plexus  split  up  into  isolated  vessels  (figs.  6,  7  and  8,  Sabin  135). 
On  the  injected  side  there  was  just  one  lymphatic  vessel  which  did  not 
receive  the  injection  mass,  and  there  was  an  extravasation  just  at  its 
base.  This  is  readily  explained  by  the  fact  that  vessels  are  often  con- 
nected by  very  slender  strands  to  the  main  plexus,  as,  for  example,  the 
vessel  near  the  point  of  injection  in  fig.  18,  and  a  rupture  might  readily 
occur  in  such  an  area  before  the  end  of  the  vessel  was  reached. 

The  test  of  the  two  methods  has  now  been  made  a  third  time  by 
Mrs.  E.  E.  Clark  (27)  in  her  injection  of  the  jugular  lymphatic 
plexus  in  a  stage  corresponding  to  the  one  which  Miller  (97)  has 
reconstructed.  The  two  results  are  shown  in  figs.  14  and  15. 

A  comparison  of  the  injected  jugular  sac  in  a  pig  18  mm.,  shown  in 
fig.  7,  with  reconstructions  of  the  corresponding  stages  in  the  cat 
(figs.  60  to  62,  Huntington  55),  also  brings  out  the  same  point  that, 
wherever  the  injection  method  can  be  applied,  it  demonstrates  more 
continuous  lymphatics  in  an  area  than  can  be  reconstructed,  even  as 
isolated  vessels. 

Undoubtedly  an  occasional  blood  capillary  or  lymphatic  capillary 
may  separate  from  the  main  plexus  and  atrophy,  but  the  apparently 
isolated  vessels  found  in  serial  sections  along  the  course  of  grow- 
ing lymphatics  connect  in  life.  Lymphatics  do  not  grow  by  de- 


The  Origin  and  Development  of  the  Lymphatic  System.       75 

tached  blood  vessels,  and  hence  the  hypothetical  Mayer-Lewis  anlagen 
do  not  exist,  that  is  to  say,  they  are  not  anlagen  of  lymphatics, 
but  are  parts  of  a  continuous  vessel.  It  is  clear  that  reconstructions  of 
growing  lymphatics  are  valuable,  even  though  they  show  the  lym- 
phatics as  rows  of  beads,  provided  only  the  true  lymphatics  are  in- 
cluded; that  is  to  say,  the  endothelial-lined  vessels.  For  example, 
Lewis  (76,  figs.  7  and  8)  shows  that  the  early  lymphatics  in  the  rabbit 
grow  along  the  thoraco-epigastric  vein. 

2.    ExTRA-IXTniAL    AXD     PERIXEURAL     SPACES     AXD     FEXESTRATIOX. 

Huntington  and  McClure  (51)  began  their  work  on  the  lymphatic 
system  in  1907  with  a  study  of  the  jugular  lymphatics  in  the  cat. 
They  thought  that  lymphatics  began  as  clefts  between  the  intima, 
the  vein  and  the  surrounding  mesenchyme,  so  that  for  a  time  the 
wall  of  a  lymphatic  was  half  venous  endothelium  and  half  mesen- 
chyme. These  spaces  they  called  "  extra-intimal "  lymphatics.  As 
far  as  the  jugular  lymphatic  sacs  were  concerned  they  subsequently 
abandoned  this  idea,  accepting  the  theory  that  the  jugular  sacs  are 
derived  from  the  veins  (Huntington  and  McClure  54),  but  they  have 
since  revived  the  extra-intimal  theory  in  connection  with  the  thoracic 
duct  (Huntington  58). 

Their  work  on  the  jugular  sacs  has  been  given  in  section  IV.  In 
genera]  they  accept  the  theory  of  the  venous  origin  of  the  jugular 
sacs,  but  they  confuse  the  picture  (1)  by  the  inclusion  of  some  peri- 
neural  spaces  (fig.  22,  Huntington  and  McClure  54)  and  (2)  by  the 
idea  of  fenestration. 

The  perineural  spaces  are  an  interesting  phenomenon.  I  con- 
sidered them  in  my  first  paper  in  1902.  Along  the  entire  central 
nervous  system  and  following  the  peripheral  nerves  are  dilated  tissue 
spaces.  Within  the  spinal  canal  these  spaces  are  the  anlage  of  the 
spaces  of  the  pia-arachnoid.  Along  the  nerves  we  call  them  perineural 
spaces.  All  sections  of  embryonic  tissue  show  them.  They  are  of 
undoubted  significance  for  the  physiology  of  the  growing  nerves,  but 
they  never  bear  any  relation  to  the  lymphatics.  Injections  of  them 
outline  the  nerves  and  never  enter  lymphatic  vessels — occasionally 
they  can  be  injected  from  the  pia-arachnoid,  but  usually  the  dense 
tissue  between  the  vertebrae  prevents  this. 

The  question  of  fenestration  was  considered  in  section  IV.  Paral- 
lel veins  are  formed  by  the  same  process  of  sprouting  of  endothelium 


76  Florence  R.  Sabin. 

by  which  all  other  vessels  are  formed,  and  the  term  fenestration, 
giving  the  vague  suggestion  of  the  splitting  of  the  wall  of  a  vein 
once  formed,  does  not  describe  the  actual  process  and  is  misleading. 
We  now  know  that  the  jugular  sacs  bud  direct!}'  from  the  anterior 
cardinal  vein  as  lymphatics  and  grow  by  the  sprouting  of  their  endo- 
thelium. 

As  has  been  said,  Huntington  and  McClure  believe  that  the  jugular 
lymphatics  come  from  the  veins,  but  that  they  remain  only  as  a  means 
of  communication  between  the  veins  and  the  lymphatic  duets.  The 
lymphatic  ducts  they  think  develop  in  a  variety  of  way?. 

The  entire  question  of  the  method  of  growth  of  the  lymphatic 
system  is  now  concentrated  on  a  study  of  the  methods  of  growth  of 
the  thoracic  duct.  It  is  probable  that  the  thoracic  duct  arises  in  two 
places;  that  it  is  formed  in  mammals  by  an  asymmetrical  down- 
growth  from  the  left  jugular  sac  and  by  a  plexus  which  arises  sym- 
metrically from  the  renal  veins  and  grows  along  the  course  of  the  aorta. 
This  second  portion  arises  later  than  the  first  and  is  in  the  blood-filled 
stage  (pig  23  mm.)  when  the  jugular  stalk  is  empty.  These  two  anlagen 
grow  as  do  all  other  lymphatics  and  join  by  the  same  process  by  which 
any  lymphatic  plexus  is  formed. 

The  opposing  views  are  brought  out  in  two  papers,  one  by  Hunting- 
ton  (58)  and  one  by  Kampmeier  (66).  Huntington  describes  a  compli- 
cated method  of  origin  of  the  thoracic  duct,  including  (1)  a  jugular 
stalk;  (2)  extra-intimal  spaces,  and  (3)  general  tissue  spaces.  Kamp- 
meier, on  the  other  hand,  gives  a  clear  and  excellent  presentation  of 
the  theory  of  the  origin  of  lymphatics  by  the  addition  of  tissue  spaces. 

I  will  begin  with  the  work  of  Huntington  (58).  His  memoir  on 
the  peripheral  lymphatics  has  excellent  photographs  of  sections,  so  that 
it  is  easy  to  see  what  he  is  considering  as  lymphatics.  For  example, 
figs.  1  to  9  are  tissue  spaces,  figs.  10  to  12  are  extra-intimal  spaces, 
and  figs.  13  to  19  are  probably  lymphatics.  Figure  25  shows  the  iliac 
sac  labeled  76.  Numerous  examples  of  extra-intimal  spaces  are  shown 
in  figs.  105  to  147.  Perhaps  the  best  figure  of  an  extra-intimal  space 
is  Xo.  111A.  Figures  148  to  158  are  lymphatics. 

The  theory  of  the  origin  of  lymphatics  from  extra-intimal  spaces 
has  been  especially  described  in  an  article  by  Huntington  in  1910 
(55).  In  a  diagram  on  page  409  he  gives  his  idea  that  a  space  which 
forms  around  a  degenerating  vein  eventually  includes  the  entire  vein, 
which  then  disappears  so  that  the  lymphatic  is  left  with  a  wall  of 
mesenchyme. 


The  Origin  and  Development  of  the  Lymphatic  System.       77 

There  are  numerous  points  which  rule  out  the  theory  of  the  origin' 
of  the  lymphatics  by  extra-intimal  spaces.  (1)  These  spaces  can  be 
varied  at  will  by  changing  the  fixation.  In  my  experience  they  are 
much  more  common  with  lymphatics  than  with  veins.  We  have  sec- 
tions of  adult  human  tonsils,  in  which,  all  surrounding  lymphatics 
have  extra-intimal  spaces.  In  the  human  embryo,  460,  the  large 
jugular  segment  of  the  thoracic  duct  has  one  area  where  the  endo- 
thelium  has  sagged  from  the  surrounding  tissue.  There  is,  I  think, 
no  question  but  that  the  extra-intimal  spaces  are  artefacts.  This  is 
made  the  more  certain  by  the  fact  that  they  do  not  occur  in  the  living 
tadpole's  tail,  but  are  found  along  the  veins  after  fixation. 

(2)  In    connection   with   the   presentation   of   the   extra-intimal 
spaces,  Huntington  gives  no  proof  whatever  that  the  structures  he 
figures  are  degenerating  veins.     Most  of  the  extra-intimal  spaces  he 
shows  are  along  the  line  of  the  pulmonary  and  cardiac  lymphatics 
arising  from  the  jugular  sacs.     I  think  it  probable  from  their  size 
and   position   that  the   structures  he   is   dealing  with   are   shrunken 
lymphatics  and  not  veins.     The  pulmonary  and  cardiac  lymphatics 
(fig.  13)  will,  when  reconstructed,  give  the  same  appearance  of  iso- 
lated lymphatic  vessels  as  all  other  lymphatic  trunks. 

(3)  The  third  point  against  the  theory  that  lymphatics  grow  by 
extra-intimal  spaces  is  this :   the  growing  lymphatic  tip  always  keeps 
as  far  as  possible  from  the  blood  capillaries,  just  as  in  the  adult  the 
ultimate  lymphatic  capillaries  are  as  far  as  possible  from  the  blood 
capillaries;  as,  for  example,  in  the  relation  of  the  central  lacteals 
and  peripheral  blood  capillaries  in  the  villus.    The  point  of  the  avoid- 
ance of  the  blood  capillaries  and  of  the  mesenchyme  cells  as  well  by 
the  growing  lymphatic  tip  has  been  well  described  by  Clark   (26). 
Moreover,  in  certain  tadpoles  the  lymphatics  grow  out  ahead  of  the 
blood  capillaries  where  no  blood  capillaries  have  ever  been.    Therefore, 
lymphatics   do  not  grow  by  extra-intimal   spaces,   for   extra-intimal 
spaces   are   artefacts;   there   is   no  evidence  that  lymphatics   follow 
degenerating  veins  and  on  the  other  hand  the  growing  lymphatic 
tip,  far  from  following  the  blood  capillaries,  avoids  them  as  much 
as  possible. 

3.  GROWTH  OF  LYMPHATICS  BY  THE  ADDITION  OF  TISSUE  SPACES. 

With  these  various  structures  as  anlagen  of  lymphatic  capillaries, 

Huntington  and  McClure  have  included  certain  of  the  tissue  spaces 

lying  along  the  course  of  the  lymphatics.     This  theory  Huntington 


78  Florence  R.  Sabin. 

brought  out  especially  in  a  study  of  the  lymphatic  system  in  reptiles 
(57).  After  a  confused  account  of  the  origin  of  the  anterior  lymph 
hearts  he  describes  the  development  of  the  peripheral  lymph  vessels 
out  of  tissue  spaces.  He  says  that  whereas  in  mammals  the  greater 
number  of  the  lymphatic  vessels  come  from  extra-intimal  spaces  (57, 
p.  272)  in  reptiles,  on  the  other  hand,  the  lymphatics  come  from 
tissue  spaces  without  relation  to  the  veins. 

The  theory  that  in  the  pathway  of  developing  lymphatic  vessels 
certain  tissue  spaces  enlarge  and  are  added  onto  the  growing  tips 
has  been  worked  out  by  two  pupils  of  McClure,  Kampmeier  (66a)  and 
Stromsen  (146a),  both  of  whom  published  their  work  in  1912. 

Kampmeier's  first  article  (66)  is  a  preliminary  report  of  the  second 
(66a).  He  has  worked  on  the  development  of  the  thoracic  duct  in  the 
pig  and  bases  most  of  his  conclusions  on  a  reconstruction  of  a  speci- 
men of  mine.  Kampmeier  thinks  that  the  thoracic  duct  develops  in 
an  anterior-posterior  direction  by  the  addition  of  certain  connective 
tissue  spaces  which  enlarge  in  the  pathway  of  a  developing  vessel. 
This  is  a  return  to  the  view  of  the  earlier  embryologists,  except  that 
they  believed  that  the  growth  was  from  the  periphery  toward  the  center. 

To  a  certain  extent  Kampmeier  still  holds  to  the  theory  of  lym- 
phatics from  extra-intimal  spaces;  that  is,  he  believes  that  a  part  of 
the  thoracic  duct  follows  veins  that  are  degenerating.  A  vein  which 
lies  in  the  pathway  of  a  developing  lymphatic  vessel  he  calls  a  "  veno- 
lymphatic."  This  is  a  different  use  of  the  term  from  that  of  Hunting- 
ton  and  (McClure,  who  used  the  term  to  mean  a  vein  which  was  trans- 
formed into  a  lymphatic  vessel.  This  latter  use  of  the  term  can  well 
be  given  up,  since  veins  do  not  become  lymphatics.  The  replacing 
of  degenerating  veins  Kampmeier  does  not  regard  as  a  fundamental 
process  in  the  development  of  lymphatics,  since  they  may  or  may  not 
follow  such  veins.  He  thinks  that  in  the  development  of  the  thoracic 
duct  about  half  the  duct  comes  from  extra-intimal  spaces  (66a,  p. 
434).  It  is  interesting  to  note  in  connection  with  the  degeneration 
of  veins  in  Kampmeier's  work  that  he  speaks  of  the  plexus  of  veins 
medial  to  the  nervous  sympatheticus,  fig.  11  (or  fig.  11  in  66a),  as 
veno-lymphatics ;  that  is,  as  veins  which  disappear  as  the  thoracic 
duct  forms,  and  yet  this  same  plexus  of  veins  can  be  readily  injected 
in  embryo  pigs  27  to  30  mm.  long;  stages  in  which  the  thoracic  duct 
is  well  formed.  It  is  true  that  the  blood  vessels  may  not  show  in 
uninjected  specimens,  just  as  the  plexus  of  blood  capillaries  around 


The  Origin  and  Development  of  the  Lymphatic  System.       79 

the  air  cells  of  an  adult  lung  cannot  be  reconstructed  from  sections 
of  uninjected  specimens.  The  degeneration  of  blood  vessels  can  only  be 
followed  by  means  of  complete  injections  of  different  stages. 

Kampmeier  finds  another  example  of  lymph  vessels  which  grow 
by  the  extra-intimal  replacement  of  veins  in  the  region  where  the 
thoracic  duct  empties  into  the  jugular  sac.  In  his  fig.  8  from  an 
embryo  20  mm.  long  he  shows  vessels  in  which  there  is  a  considerable 
sagging  of  the  endothelium  from  the  surrounding  tissue,  making  the 
so-called  extra-intimal  spaces.  Kampmeier's  embryo  measuring  20 
mm.  is  about  the  same  stage  as  mine,  measuring  23  mm.,  since 
mine  was  measured  before  fixation,  and  hence  his  figure  can  be  com- 
pared with  fig.  12,  in  which  a  lymphatic  injection  makes  it  easy  to 
separate  veins  from  lymphatics.  As  Hoyer  says  (49b,  p.  536)  :  "Den 
Yerlauf  der  Lymphgefasse  nur  an  nicht  injizierten  Serienschnitten  zu 
verfolgen,  ist  eine  muhevolle  und  zeitraubende  Arbeit,  deren  Ergeb- 
nisse,  wie  wir  uns  selbst  iiberzeugt  haben,  hochstens  nur  fiir  grossere 
Lymphstamme,  deren  Lage  man  kennt,  als  sicher  zu  bezeichnen  sind. 
Ist  der  Yerlauf  von  Gefassen  und  deren  Yerastelung  noch  unbekannt, 
so  lassen  sich  auf  Grund  von  Serienschnitten  nur  Yermutungen 
iiber  Yerlauf  und  Yerastelung  derselben  austellen.  Erst  wenn  man 
iiber  Gefasse  und  ihre  Aeste  mit  Injektionsmasse  gefiillt  vor  sich  sieht, 
erhalt  man  einen  guten  Einblick  in  die  Yerteilung  derselben,  deren 
Yerfolgung  selbst  an  nicht  injizierten  Serienschnitten  dann  keine 
wesentlichen  Schwierigkeiten  mehr  bietet."  Although  it  is  not  pos- 
sible to  be  entirely  sure  in  regard  to  Kampmeier's  sections,  yet  it  is 
at  least  probable  that  he  is  dealing  in  large  part  with  lymphatics  and 
not  with  veins  at  all.  The  sagging  of  the  endothelium  from  the 
surrounding  tissue  I  have  found  more  often  with  lymphatics  than 
with  veins,  and  yet  in  tissues  fixed  in  Zeuker's  fluid,  which  Kamp- 
meier uses  exclusively,  it  may  occur  in  any  of  the  veins  and  even  in 
the  aorta.  Kampmeier  says,  with  reference  to  this  group  of  vessels 
(fig.  8  and  fig.  29  in  86a,  p.  460),  that  they  are  mesenchymal,  peri- 
vascular  spaces  into  which  the  jugular  sac  opens  freely;  that  is,  he 
thinks  that  the  jugular  sac  in  an  embryo  20  mm.  long  opens  freely 
into  the  tissue  spaces.  This  point  is  easy  to  disprove,  for  in  contrast, 
to  the  early  stages  of  the  thoracic  duct  the  jugular  sac  in  this  stage 
is  easy  to  inject.  If  this  were  true  then  every  injection  of  the  sac  in 
this  stage  would  show  extravasations.  This  is,  however,  not  so.  In 
fig.  7  is  an  injection  of  the  sac  in  an  embryo  18  mm.  long  and  I  can 


80  Florence  R.  Salin. 

state  with  great  certainty  on  the  basis  of  many  injections  that  the 
jugular  sac  has  a  complete  endothelial  wall  and  is  a  closed  vessel. 
It  is  thus  clear  that  Kampmeier  has  not  demonstrated  that  this  upper 
part  of  the  thoracic  duct  replaces  degenerating  veins. 

The  chief  point,  however,  in  Kampmeier's  work  is  not  that  some 
of  the  lymphatics  replace  degenerating  veins,  but  that  they  develop 
out  of  tissue  spaces.  As  the  chief  proof  of  this  theory  he  uses  a 
reconstruction  of  an  injected  specimen  of  mine.  Notwithstanding  the 
fact  that  this  injection  is  the  only  one  which  has  yet  been  made  in  a 
mammal  in  this  early  stage,  Kampmeier  does  not  hesitate  to  call  it 
complete.  It  was  injected  indirectly  through  the  jugular  lymph  sac, 
from  which  the  injection  mass  ran  into  the  thoracic  duct.  At  a  cer- 
tain point  in  the  injection  there  is  an  extravasation  (Kampmeier, 
fig.  13,  line  15),  and  in  exactly  the  same  position  in  the  next  section 
is  a  large  endothelial-lined  empty  space.  It  is  therefore  merely  an 
arbitrary  decision  whether  the  empty  vessel  was  actually  connected 
with  the  injected  part  or  not,  that  is  to  say,  there  is  as  much  evidence 
for  the  one  view  as  for  the  other.  No  one  who  has  had  experience  with 
the  injection  method  would  be  sure  that  the  first  injection  in  a  new 
region  was  a  complete  one.  The  spaces  which  Kampmeier  has  shown 
as  lymphatics  in  my  specimen  are  lined  by  endothelium:  that  is. 
they  are  the  spaces  with  which  Lewis  (76)  has  made  us  familiar; 
they  are  not  the  mesenchyme  spaces  which  Kampmeier  and  Strom- 
sen  regard  as  the  anlage  of  lymphatics. 

Stromsen  (146a)  has  injected  the  prsvertebral  lymphatics  in  turtles 
and  finds  that  in  advance  of  the  injected  zone  there  are  enlarged 
tissue  spaces  which  he  thinks  are  going  to  become  lymphatics.  Kamp- 
meier says  that  it  is  easy  to  select  tissue  spaces  which  are  going  to 
become  lymphatics  for,  "  histologically,  all  incipient  lymphatic  an- 
lagen,  whether  they  are  spaces  independent  in  position  or  spaces  fol- 
lowing, transforming  and  expanding  the  discarded  pathways  of 
redundant  venous  channels,  are  decidedly  different  from  either  an 
active  vein  or  a  mature  lymphatic  "  (Kampmeier  G6a,  p.  430) .  Strom- 
sen  (146,  p.  354)  adds  to  this  ability  to  select  tissue  spaces  which  are 
going  to  become  lymphatics  this  further  point,  that  such  enlarged  tis- 
sue space  occur  only  in  the  pathway  of  developing  lymphatics.  These 
two  points  can  be  easily  disproved  by  anyone  who  has  access  to  serial 
sections  of  embryos.  For  example,  why  were  not  the  enlarged  tissue 
spaces  in  Kampmeier  fig.  3  selected  as  lymphatics?  Good  examples 


Tlie  Origin  and  Development  of  the  Lymphatic  System.       81 

of  such  spaces  are  the  constantly  occurring  spaces  in  the  parietal 
pleura  on  either  side  of  the  vertebra.  They  are  always  present  in  the 
stages  in  which  the  thoracic  duct  is  developing;  they  are  adjacent  to 
the  vascular  zone  just  internal  to  the  ribs  and  they  never  have  any 
relation  to  lymphatics.  A  definite,  constant  group  of  such  spaces  is 
also  to  be  found  in  the  subcutaneous  tissue  of  the  mid-dorsal  line  of 
certain  stages  and  they  never  become  lymphatics.  The  pia-arachnoid 
and  perineural  spaces  likewise  never  become  lymphatics.  In  fact, 
large  as  well  as  small  groups  of  such  spaces  are  a  constant  occurrence 
in  sections  of  embryos;  sometimes  near  lymphatics  and  sometimes  far 


YIG.  19. — Diagram  to  show  some  of  the  various  structures  which  Hunt- 
ington  and  McClure  have  described  as  lymphatics,  a.  =  true  lymphatic 
capillaries;  b.  =  Mayer-Lewis  anlagen,  also  true  lymphatic  capillaries;  c. 
=  extra-intimal  hypothetical  lymphatic  capillaries;  d.  =  tissue  spaces. 
The  perineural  spaces  are  not  shown.  The  veins  are  striped,  lymphatic 
capillaries  clotted,  and  hypothetical  lymphatic  capillaries  are  cross-hatched. 

from  them.  The  selection  of  certain  tissue  spaces  as  destined  to 
become  lymphatics  is  an  arbitrary  selection  and  brings  us  back  to  the 
confused  standpoint  of  the  earlier  embryologists ;  a  standpoint  which 
would  indeed  justify  the  view  of  Bartels  that  the  question  of  the 
relation  of  the  lymphatics  to  the  tissue  spaces  is  "  eine  philosophische, 
keine  anatomische  Frage." 

The  difficulty  of  finding  out  the  method  of  growth  by  observing 
sections  is  best  illustrated  by  following  the  different  theories  of  their 
growth  in  the  pages  of  Huntington  and  McClure.  Some  of  these 
methods  are  shown  in  the  diagram,  fig.  19.  This  development  of  the 


82  Florence  R.  Sabin. 

subject  has  been  necessarily  confusing  to  those  who  have  not  followed 
the  subject  carefully.  However,  now  that  the  origin  and  method  of 
growth  of  the  lymphatic  system  has  been  cleared  up,  so  that  the  funda- 
mental morphology  is  understood,  the  controversy  has  had  this  great 
value,  that  it  has  brought  up  for  analysis  and  discussion  every  con- 
ceivable method  of  growth.  Lymphatics  do  not  arise  as  dilated  per- 
ipheral tissue  spaces  after  the  manner  of  the  ccelom  as  the  earlier 
embryologists  thought ;  they  do  not  grow  by  the  addition  of  hollow  con- 
nective tissue  cells,  as  Schwann  and  Virchow  thought ;  they  do  not  rise 
as  perineural  spaces,  nor  by  fenestration  of  a  vein,  nor  by  extra-intimal 
clefts,  nor  by  the  progressive  addition  of  connective  tissue  spaces,  nor 
by  the  addition  of  detached  blood  vessels,  but  they  bud  from  the  veins 
and  grow  by  the  sprouting  of  their  endothelial  wall. 

IX.  CONCLUSIONS. 

The  most  important  result  of  this  study  on  the  morphology  of  the 
lymphatic  system  is  the  emphasis  it  throws  on  the  importance  of 
endothelium  as  a  tissue.  The  angioblast  is  one  of  the  early  tissues 
to  be  differentiated ;  it  is  not  an  inert  lining  for  vessels,  but  an  actively 
growing  functioning  tissue.  In  its  place  of  growth  it  is  a  syncytium 
of  actively  amceboid  protoplasm.  Mollier  (99)  has  shown  that  in  the 
spleen  it  may  be  reticular;  Mall  (86  and  87)  has  shown  that  it  may 
give  rise  to  reticulum.  Undoubtedly  the  further  development  of  our 
knowledge  of  endothelium  depends  on  the  development  of  the  new 
experimental  anatomy. 

The  lymphatic  endothelium  buds  off  from  the  veins.  It  is  always 
a  little  different  in  appearance  from  the  endothelium  of  the  veins,  and 
the  lymphatic  capillary  is  different  in  size  and  form  from  the  blood 
capillaries.  The  growing  lymphatic  tip  has  the  remarkable  character- 
istic that  it  avoids  the  blood  capillaries,  while  it  is  attracted  by  other 
lymphatic  capillaries. 

Endothelium  is  the  essential  tissue  of  the  lymphatic  system.  In  the 
lower  vertebrates  lymph  hearts  are  formed  by  the  addition  of  striated 
muscle  to  primary  lymph  sacs.  In  the  higher  forms  lymph  glands 
are  formed  by  the  development  of  lymphocytes  around  the  ducts. 
This  takes  place  not  only  in  the  wall  of  the  primary  lymph  sacs,  but 
along  plexuses  of  ducts,  so  that  there  are  primary  and  secondary 
lymph  glands. 


The  Origin  and  Development  of  the  Lymphatic  System.       S3 

The  fundamental  morphology  of  the  lymphatic  system  has  been  es- 
tablished, but  there  remain  many  gaps  in  our  knowledge  of  the  system 
as  a  whole.  The  deep  lymphatics  in  amphibia  and  reptilia  are  almost 
unknown ;  in  fact  a  complete  account  of  the  lymphatic  systems  in 
both  of  these  groups  would  be  of  great  value.  In  birds  the  origin  of 
the  iliac  lymphatics  and  the  growth  of  the  thoracic  duct  would  be  most 
interesting.  In  mammals  the  gaps  in  our  knowledge  are  especially 
in  regard  to  the  development  of  the  lymphatic  system  within  many 
of  the  organs;  for  example,  the  heart  and  lungs,  the  liver,  spleen, 
kidneys  and  reproductive  organs.  A  study  of  the  embryology  of  the 
lymphatic  system  makes  it  more  certain  that  the  central  nervous 
system  has  no  lymphatics.  The  lymphatics  invade  the  body,  but  not 
completely;  the  nervous  system  is  a  permanent  non-lymphatic  area. 

It  is  now  possible  to  define  the  lymphatics.  Lymphatic  capillaries 
are  tubes  of  endothelium;  they  are  derived  from  the  endothelium  of 
the  veins,  and  they  have  the  same  relation  to  tissue  spaces  as  have 
blood  capillaries. 


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